Feline hyperthyroidism: Difference between revisions
Traumnovelle (talk | contribs) No edit summary |
ShelfSkewed (talk | contribs) →Causes: dab link |
||
| (38 intermediate revisions by 10 users not shown) | |||
| Line 1: | Line 1: | ||
{{Short description| |
{{Short description|Endocrine disorder in cats}} |
||
{{Infobox medical condition |
|||
|name = Hyperthyroidism |
|||
|synonyms = <!-- or |synonym= --> |
|||
|image = |
|||
|image_upright = |
|||
|image_size = |
|||
|alt = |
|||
|image_thumbtime = |
|||
|caption = |
|||
|width = |
|||
|image2 = |
|||
|image2_upright = |
|||
|image_size2 = |
|||
|alt2 = |
|||
|image_thumbtime2 = |
|||
|caption2 = |
|||
|width2 = |
|||
|pronounce = |
|||
|pronounce 2 = |
|||
|specialty = [[Endocrinology]] |
|||
|symptoms = Weight loss, [[polyphagia]], [[polydipsia]], [[tachycardia]], [[polyuria]], restlessness |
|||
|complications = Cardiac problems, [[cachexia]], [[hypertension]] |
|||
|onset = Average 13 years |
|||
|duration = Lifelong if not properly treated |
|||
|types = |
|||
|causes = Tumour, with multifactorial cause behind it. |
|||
|risks = Age, diet, chemicals, non-[[Siamese cat|Siamese]] breeds, indoor cats. |
|||
|diagnosis = Measurement of serum [[thyroxine]] levels |
|||
|differential = [[Diabetes mellitus]] |
|||
|prevention = Limiting iodine intake |
|||
|treatment = Thyroidectomy, radioactive iodine, anti-thyroid drugs |
|||
|medication = Methimazole, carbimazole |
|||
|prognosis = Average life expectancy of 15 years if treated |
|||
|frequency = Most common endocrinopathy in cats |
|||
|deaths = |
|||
|named after = <!-- or |eponym= --> |
|||
}} |
|||
'''Feline hyperthyroidism''' is an [[endocrine disorder]] in which the thyroid gland produces too much [[thyroid hormone]]. Hyperthyroidism is the most common [[endocrinopathy]] of cats. The complete [[pathogenesis]] is not fully understood. |
'''Feline hyperthyroidism''' is an [[endocrine disorder]] in which the thyroid gland produces too much [[thyroid hormone]]. Hyperthyroidism is the most common [[endocrinopathy]] of cats. The complete [[pathogenesis]] is not fully understood. |
||
| Line 10: | Line 48: | ||
The majority of cases of hyperthyroidism in cats are the result of benign tumours. The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are [[Thyroid follicular cell|follicular cell]] [[adenoma]] and [[Nodule (medicine)|multinodular]] adenomatous [[hyperplasia]]. 1% to 3% of feline hyperthyroid cases are caused by [[malignant tumour]]s. These are not always able to be distinguished from benign tumours.<ref name="endo"/> |
The majority of cases of hyperthyroidism in cats are the result of benign tumours. The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are [[Thyroid follicular cell|follicular cell]] [[adenoma]] and [[Nodule (medicine)|multinodular]] adenomatous [[hyperplasia]]. 1% to 3% of feline hyperthyroid cases are caused by [[malignant tumour]]s. These are not always able to be distinguished from benign tumours.<ref name="endo"/> |
||
It is believed that the cause of hyperthyroidism is [[multifactorial]]. Studies have identified different mutations as causing thyroid cell autonomy.<ref name="endo">{{cite book |author1-first=J. Catherine |author1-last=Scott-Moncrieff |editor1-last=Feldman | editor1-first=Edward C. | editor2-last=Nelson | editor2-first=Richard W. | editor3-last=Reusch | editor3-first=Claudia | editor4-last=Scott-Moncrieff | editor4-first=J. Catharine |title=Canine and feline endocrinology |date=2015 |publisher=Elsevier Saunders |location=St. Louis, Missouri |isbn=978-1-4557-4456-5 |edition=Fourth |url=http://www.sciencedirect.com/book/9781455744565/canine-and-feline-endocrinology |chapter=Feline Hyperthyroidism| publication-place=St. Louis, Missouri | pages=137–190}}</ref> |
It is believed that the cause of hyperthyroidism is [[Quantitative trait locus#Multifactorial traits in general|multifactorial]]. Studies have identified different mutations as causing thyroid cell autonomy.<ref name="endo">{{cite book |author1-first=J. Catherine |author1-last=Scott-Moncrieff |editor1-last=Feldman | editor1-first=Edward C. | editor2-last=Nelson | editor2-first=Richard W. | editor3-last=Reusch | editor3-first=Claudia | editor4-last=Scott-Moncrieff | editor4-first=J. Catharine |title=Canine and feline endocrinology |date=2015 |publisher=Elsevier Saunders |location=St. Louis, Missouri |isbn=978-1-4557-4456-5 |edition=Fourth |url=http://www.sciencedirect.com/book/9781455744565/canine-and-feline-endocrinology |chapter=Feline Hyperthyroidism| publication-place=St. Louis, Missouri | pages=137–190}}</ref> |
||
===Iodine=== |
===Iodine=== |
||
Excessive [[iodine]] administration has been hypothesised as causing [[thyrotoxicosis]] in cats, as it does in humans; however, cats have been shown in multiple long terms studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. [[Iodide]] intake and |
Excessive [[iodine]] administration has been hypothesised as causing [[thyrotoxicosis]] in cats, as it does in humans; however, cats have been shown in multiple long terms studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. [[Iodide]] intake and its effects are unknown.<ref name="endo"/> |
||
Iodine amounts in cat food vary by as much as a factor of 30. Low, high, and variable iodine diets have been hypothesised as cause of feline hyperthyroidism.<ref name="endo"/> |
|||
===Soy isoflavones=== |
===Soy isoflavones=== |
||
[[Soy]] is often used in commercial cat food diets as a vegetable protein. In one study more than half of commercial cat foods surveyed contained [[soy isoflavones]]. [[Genistein]] and [[daidzein]], both of which occur in soybeans, inhibit the enzymes [[thyroid peroxidase]] and [[thyroxine 5-deiodinase]]. This causes decreased [[thyroxine]] and [[triiodothyronine]] concentrations. In response to decreased triiodothyronine levels the body will produce more [[thyroid-stimulating hormone]] to normalise triiodothyronine levels, this has been shown to result in increased thyroxine levels. In addition these effects are heightened when a cat is suffering from iodine deficiency. However, further research is needed to confirm a link between soy isoflavones and hyperthyroidism.<ref name="endo"/> |
[[Soy]] is often used in commercial cat food diets as a vegetable protein. In one study more than half of commercial cat foods surveyed contained [[soy isoflavones]]. [[Genistein]] and [[daidzein]], both of which occur in soybeans, inhibit the enzymes [[thyroid peroxidase]] and [[thyroxine 5-deiodinase]]. This causes decreased [[thyroxine]] and [[triiodothyronine]] concentrations. In response to decreased triiodothyronine levels the body will produce more [[thyroid-stimulating hormone]] to normalise triiodothyronine levels, this has been shown to result in increased thyroxine levels. In addition these effects are heightened when a cat is suffering from iodine deficiency. However, further research is needed to confirm a link between soy isoflavones and hyperthyroidism.<ref name="endo"/> |
||
===Goitrogens=== |
===Goitrogens=== |
||
[[Goitrogens]] are chemicals that disrupt thyroid hormone production. Chemicals such as [[bisphenol A]] and [[polybrominated diphenyl ethers]] have been shown to exist in indoor cats at heightened levels and are hypothesised to be a potential cause of hyperthyroidism. Bisphenol A is common in canned cat foods, although no association between it and hyperthyroidism has currently been demonstrated. Polybrominated diphenyl ethers (PBDEs) are often used as fire retardants and are known to be a goitrogen; however, levels of these chemicals have been shown to be the same in euthyroid cats as hyperthyroid cats. No association has been proven in a study for the association between PBDEs and hyperthyroidism.<ref name="endo"/> |
[[Goitrogens]] are chemicals that disrupt thyroid hormone production. Chemicals such as [[bisphenol A]] and [[polybrominated diphenyl ethers]] have been shown to exist in indoor cats at heightened levels and are hypothesised to be a potential cause of hyperthyroidism. Bisphenol A is common in canned cat foods, although no association between it and hyperthyroidism has currently been demonstrated. Polybrominated diphenyl ethers (PBDEs) are often used as fire retardants and are known to be a goitrogen; however, levels of these chemicals have been shown to be the same in euthyroid cats as hyperthyroid cats. No association has been proven in a study for the association between PBDEs and hyperthyroidism.<ref name="endo"/> |
||
| Line 32: | Line 75: | ||
Cardiac conditions have become less common over the years in hyperthyroid cats due to earlier diagnosis and improved treatment. The most common cardiovascular issues are [[tachyarrhythmias]], [[cardiac murmur]]s, and [[gallop rhythm]]s. The cause of these conditions is due to how [[triiodothyronine]] increases [[cardiac output]].<ref name="endo"/> |
Cardiac conditions have become less common over the years in hyperthyroid cats due to earlier diagnosis and improved treatment. The most common cardiovascular issues are [[tachyarrhythmias]], [[cardiac murmur]]s, and [[gallop rhythm]]s. The cause of these conditions is due to how [[triiodothyronine]] increases [[cardiac output]].<ref name="endo"/> |
||
Thyroid [[cyst]]s can occur in cases of adenoma, adenomatous hyperplasia, or carcinoma. These cysts have a high concentration of thyroxine. Cysts should not be treated with radioactive iodine and instead be surgically removed.<ref name="endo"/> |
|||
Symptoms of [[Cushing's syndrome (veterinary)|hyperadrenocorticism]] can occur in hyperthyroid cats. [[Adrenocorticotropic hormone]] levels are raised in some hyperthyroid cats and some studies have documented [[acromegaly]] in hyperthyroid cats.<ref name="endo"/> |
Symptoms of [[Cushing's syndrome (veterinary)|hyperadrenocorticism]] can occur in hyperthyroid cats. [[Adrenocorticotropic hormone]] levels are raised in some hyperthyroid cats and some studies have documented [[acromegaly]] in hyperthyroid cats.<ref name="endo"/> |
||
===Risk factors=== |
===Risk factors=== |
||
Multiple case control studies have looked at diet and hyperthyroidism. These studies have found associations between commercial diets and hyperthyroidism. Other risk factors identified include non-[[Siamese cat|Siamese]] related breeds, lack of outdoor access, flea medication, pesticides, certain cat litters, female sex, sleeping on the floor, organic fertiliser, human baby food, carpet cleaners, [[natural gas]], lack of [[deworming]], and a fish diet.<ref>{{cite journal | last=Edinboro | first=Charlotte H | last2=Scott-Moncrieff | first2=J Catharine | last3=Glickman | first3=Larry T | title=Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods | journal=Journal of Feline Medicine and Surgery | volume=12 | issue=9 | date=2010 | issn=1098-612X | doi=10.1016/j.jfms.2010.07.011 | pages=672–679}}</ref> These mixed results suggest a multifactoral cause with diet being having an important role.<ref name="endo"/> |
Multiple case control studies have looked at diet and hyperthyroidism. These studies have found associations between commercial diets and hyperthyroidism. Other risk factors identified include non-[[Siamese cat|Siamese]] related breeds, lack of outdoor access, flea medication, pesticides, certain cat litters, female sex, sleeping on the floor, organic fertiliser, human baby food, carpet cleaners, [[natural gas]], lack of [[deworming]], and a fish diet.<ref>{{cite journal | last=Edinboro | first=Charlotte H | last2=Scott-Moncrieff | first2=J Catharine | last3=Glickman | first3=Larry T | title=Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods | journal=Journal of Feline Medicine and Surgery | volume=12 | issue=9 | date=2010 | issn=1098-612X | doi=10.1016/j.jfms.2010.07.011 | pages=672–679| pmc=11149000 }}</ref> These mixed results suggest a multifactoral cause with diet being having an important role.<ref name="endo"/> |
||
===Age of onset=== |
===Age of onset=== |
||
| Line 44: | Line 89: | ||
Diagnosing hyperthyroid cats is not just to confirm the presence of the condition but also what symptoms are present. For example more serious cardiac conditions require special treatment compared to a cat without any serious cardiac abnormalities. Cats with hyperthyroidism should have a [[complete blood count]] test, a [[biochemistry profile]] of [[blood serum]], [[urinalysis]], measurement of thyroxine serum levels, [[thoracic]] [[radiography]], and measurement of blood pressure. If heart disease is suspected [[electrocardiography]] and [[echocardiography]] should be performed.<ref name="endo"/> |
Diagnosing hyperthyroid cats is not just to confirm the presence of the condition but also what symptoms are present. For example more serious cardiac conditions require special treatment compared to a cat without any serious cardiac abnormalities. Cats with hyperthyroidism should have a [[complete blood count]] test, a [[biochemistry profile]] of [[blood serum]], [[urinalysis]], measurement of thyroxine serum levels, [[thoracic]] [[radiography]], and measurement of blood pressure. If heart disease is suspected [[electrocardiography]] and [[echocardiography]] should be performed.<ref name="endo"/> |
||
Measurement serum thyroxine levels is the best method of confirming hyperthyroidism. Measurement of serum concentrations [[triiodothyronine]] are not useful for diagnosing hyperthyroidism and neither is measurement of serum thyroid-stimulating hormone levels. Thyroxine levels on their own should not be used to diagnose hyperthyroidism: if a patient is displaying no symptoms of hyperthyroidism but test results show heightened serum thyroxine levels the possibility of other causes such as an error in the test have to be considered and excluded before establishing a diagnosis.<ref name="endo"/> |
|||
=== Complete blood count === |
=== Complete blood count === |
||
| Line 54: | Line 101: | ||
===Blood urea and creatinine=== |
===Blood urea and creatinine=== |
||
In roughly 10% of hyperthyroid cats serum [[creatinine]] levels are increased to the point of [[ |
In roughly 10% of hyperthyroid cats serum [[creatinine]] levels are increased to the point of [[azotaemia]]. In 10-20% of cats increased serum concentration of [[blood urea nitrogen]] is observed. Although the prevalence of hyperthyroid cats with [[chronic kidney disease]] is higher than this due to how hyperthyroidism results in an increase to the [[glomerular filtration rate]]. Most cats with chronic kidney disease and hyperthyroidism have the kidney disease go undiagnosed until after treatment for hyperthyroidism has begun. Between 15% and 49% of non-azotaemic hyperthyroid cats will develop azotaemia following treatment for hyperthyroidism. Further complicating the diagnosis is the shared symptoms between the conditions. It is not currently known whether the effects of hyperthyroidism cause or worsen renal disease. Chronic kidney disease is not an uncommon condition in elderly cats further complicating determination of a relationship between the two conditions.<ref name="endo"/> |
||
===Urinalysis=== |
===Urinalysis=== |
||
| Line 62: | Line 109: | ||
[[Proteinuria]] is reported in 75–80% of cats with hyperthyroidism. Likely causes of this are [[glomerular hyperfiltration]] and [[glomerular]] [[hypertension]], and structural changes in the glomerular filtration barrier.<ref name="endo"/> |
[[Proteinuria]] is reported in 75–80% of cats with hyperthyroidism. Likely causes of this are [[glomerular hyperfiltration]] and [[glomerular]] [[hypertension]], and structural changes in the glomerular filtration barrier.<ref name="endo"/> |
||
===Radiography=== |
|||
[[Thoracic]] [[radiograph]]s do not help diagnose hyperthyroidism, instead they help diagnose concurrent illnesses like thoracic neoplasia. If [[respiratory distress]], [[tachypnoea]], [[tachycardia]], [[arrhythmia]], or [[heart murmur]] are observed then thoracic radiographs should be taken. Congestive heart failure is uncommon. The most common finding is [[cardiomegaly]].<ref name="endo"/> |
|||
===Echocardiography=== |
|||
The most common [[echocardiographic]] abnormalities in hyperthyroid cats is [[hypertrophy]] of the [[left ventricular]] [[Caudal_(anatomical_term)|caudal]] wall and [[interventricular septum]]. The effects of thyroid hormone are known and explain most cardiac abnormalities;<ref name="endo"/> however, one study found only 37% of hyperthyroid cats to have an echocardiographic abnormality, with 32% having abnormalities following treatment.<ref>{{cite journal | last=Weichselbaum | first=Ralph C. | last2=Feeney | first2=Daniel A. | last3=Jessen | first3=Carl R. | title=RELATIONSHIP BETWEEN SELECTED ECHOCARDIOGRAPHIC VARIABLES BEFORE AND AFTER RADIOIODINE TREATMENT IN 91 HYPERTHYROID CATS | journal=Veterinary Radiology & Ultrasound | volume=46 | issue=6 | date=2005 | issn=1058-8183 | doi=10.1111/j.1740-8261.2005.00099.x | pages=506–513}}</ref> |
|||
===Clinical examination=== |
|||
Multiple clinical signs that support a diagnosis can be observed in hyperthyroid cats during a clinical exam. The most useful symptoms for this is weight loss with concurrent polyphagia and [[tachycardia]].<ref name="endo"/> |
|||
A [[goitre]] is swelling from an enlarged thyroid gland. This does not occur in all cases of hyperthyroidism, even those caused by adenoma; however it occurs in most cases and is a common way to support a hyperthyroid diagnosis caused by adenoma. Other conditions can cause a cervical mass that may resemble a goitre.<ref name="endo"/> |
|||
The initial step should be to determine the serum concentration of thyroxine (T4). However, in veterinary medicine, the total thyroxine concentration is typically determined, rather than that of free (non-[[protein]]-bound) thyroxine (fT4). Nevertheless, the latter is more sensitive.<ref>M. E. Peterson: ''Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease.'' In: ''J. Am. Vet. Med. Assoc.'' 218(4), 2001, p. 529–536. {{PMID|11229503}}.</ref> The normal range for T4 in cats is between 1.1 and 4.5 μg/dL, for fT4 between 1.0 and 2.8 ng/dL when determined by equilibrium dialysis.<ref name=":0">R. Hämmerling: ''Die feline Hyperthyreose.'' In: ''Der praktische Tierarzt.'' 86(5), 2005, p. 320–324. {{ISSN|0032-681X}}</ref> In approximately 20% of animals, the T4 level is normal despite the presence of disease, which may be due to fluctuations in the hormone level during the course of the day or a reduction in the T4 level as a result of other secondary diseases. The measurement of fT4 has a sensitivity of 95%, but 20 to 30% of thyroid-healthy cats also have elevated fT4 levels. Therefore, if fT4 levels are elevated, total T4 must also be measured, which should be in the upper reference range in animals with hyperthyroidism.<ref>Stephanie Nather et al.: ''Hyperthyreose der Katze.'' In: ''Kompendium Kleintier'' 2017, p. 52–57.</ref> Furthermore, various medications, including [[glucocorticoid]]s, [[Nonsteroidal anti-inflammatory drug|non-steroidal anti-inflammatory drugs]] (NSAIDs), [[phenobarbital]], and [[trimethoprim]]-[[sulfonamide]] combinations, can influence T4 levels.<ref name=":1">Eva Höfel, Thomas Rieker: ''Hyperthyreose – Aktuelles zur Pathogenese und Diagnose.'' In: ''Fachpraxis.'' No. 63, 2013, p. 6–11.</ref> In the event of a clinical suspicion, it is advisable to repeat the determination at a later date. |
|||
===Thyrotropin-releasing hormone stimulation=== |
|||
Following a test for blood serum thyroxine levels [[thyrotropin-releasing hormone]] can be administered to cats intravenously and after 4 hours another blood test can be taken to measure thyroxine serum levels. If the thyroxine levels have increased this indicates the cat is euthyroid but if levels remain stagnant it suggests hyperthyroidism. This test is rarely performed due to side effects. The most common side effects being: [[emesis]], [[tachypnoea]], and [[defecation]].<ref name="endo"/> |
|||
===Scintigraphy=== |
|||
[[Scintigraphy]] helps to show information on the thyroid gland and tumours affecting it. It is useful for identifying the size of the tumour, whether it is unilateral or bilateral, and how much radiation is required for treatment. [[Iodine-131]], [[Iodine-123]], and [[technetium-99m pertechnetate]] are all used for this. Pertechnetate is the most commonly used due its low cost, quick uptake, and short [[half-life]]. Scintigraphy can also be used to confirm hyperthyroidism when clinical symptoms are present but serum thyroxine levels remain within reference. Drugs such as [[methimazole]] and anaesthetics can influence results if not discontinued before scintigraphy.<ref name="endo"/> |
|||
===Ultrasonography=== |
|||
[[Ultrasonography]] is an alternative option to scintigraphy. Ultrasounds provide less information than scintigraphy but is easier and cheaper to conduct. Ultrasonography allows for the state of the thyroid gland to be evaluated and for an estimation of the volume but does not allow for evaluation of tissue.<ref name="endo"/> |
|||
===Differential diagnosis=== |
|||
Polyphagia and weight loss are two concurrent symptoms that have multiple causes, including: [[Diabetes in cats|diabetes mellitus]], [[malnutrition]], [[malabsorption]], and [[maldigestion]]. If these symptoms occur serum thyroxine levels should be tested to confirm or exclude a diagnosis of hyperthyroidism.<ref name="endo"/> |
|||
===Carcinoma=== |
|||
Thyroid carcinoma has largely the same symptoms as benign tumours. [[Hypercalcaemia]] may be seen in a patient with carcinoma. Symptoms that may be identified via radiography include: [[cardiomegaly]], signs of [[congestive heart failure]], signs of [[Metastasis|pulmonary metastasis]], and [[mediastinal mass]]es. The only way to definitively diagnose a thyroid carcinoma is via histopathological examination of the affected tissue. If treatment has failed thyroid carcinoma should be considered as a possibility.<ref name="endo"/> |
|||
==Treatment== |
|||
Hyperthyroidism can be either managed with [[anti-thyroid]] drugs or restricting [[iodine]] intake, or it can be treated with [[radioactive iodine]] or [[thyroidectomy]] for a permanent solution. Choice of treatment depends on a multitude of factors including cost, availability of the treatments, and age and health of the patient. In areas where available, radioactive iodine treatment is the preferred method of treating hyperthyroidism due to the low risks and high efficacy.<ref name="endo"/> |
|||
===Surgery=== |
|||
Surgery is a cheaper option than radioactive iodine treatment and does not require any equipment not found in a standard veterinary clinic. It also results in a quicker decrease in thyroid hormone levels. Surgery however can be more risky to elderly cats given the increased risk of [[anaesthetic]] complication. A bad surgery can result in nerve damage or fail to fully remove all the abnormal tissue. It may also result in [[iatrogenic]] [[Hypothyroidism in dogs#Cats|hypothyroidism]] and [[hypoparathyroidism]].<ref name="endo"/> |
|||
Surgery has become a less common treatment option with improvement of access to radioactive iodine treatment and due to risks. Risks include: [[Ectopia (medicine)|ectopic]] tissue being leftover, [[hypothyroidism]], [[hypoparathyroidism]]. Less often [[laryngeal nerve]] damage and [[Horner's syndrome]] may occur. These risks can be prevented with a more experienced surgeon and proper imaging of the thyroid gland before removal.<ref name="endo"/> |
|||
Another method is the thyroid suppression test. In this procedure, a synthetic [[triiodothyronine]] (T3, typically Liothyronine) is administered to the cat over two days. A cat with a healthy thyroid gland will respond to this treatment by reducing the secretion of [[Thyroid-stimulating hormone|TSH]] (negative feedback), which will result in a decline in the T4 concentration. However, as the hyperthyroidism has already led to a permanently low TSH level, the administration of T3 in sick cats does not result in a reduction in TSH and T4.<ref name=":3">M. E. Peterson: ''Hyperthreodism.'' In: Stephen J. Ettinger, Edward C. Feldman: ''Textbook of veterinary internal medicine.'' 5th edition. Volume 2, Saunders, 2000, {{ISBN|0-7216-7256-6}}, pp. 1400-1419.</ref> |
|||
Thyroidectomy allows for shorter hospitalisation time and for histopathological evaluation of neoplastic tissue. Thyroidectomy is not recommended when the disease is bilateral.<ref name="endo"/> |
|||
Another diagnostic procedure is the TRH stimulation test. In this test, the cat is administered [[thyrotropin-releasing hormone]] (TRH), which leads to a significant increase in the T4 concentration in healthy cats. In diseased animals, on the other hand, there is no or at most a slight increase.<ref>M. E. Peterson et al.: ''Use of the thyrotropin releasing hormone stimulation test to diagnose mild hyperthyreodism in cats.'' In: ''J. Vet. Intern. Med.'' No. 4, 1999, p. 279–286. {{ISSN|0891-6640}}</ref> However, this test sometimes has considerable side effects in cats (salivation, vomiting, palpitations, defecation), which is why it is rarely used. The TSH test, which determines the serum content of thyrotropin, a hormone that regulates the thyroid gland, is now also available for cats.<ref name=":0" /> As in humans, early forms of hyperthyroidism can be detected based on low or unmeasurable TSH levels. However, the TSH stimulation test, which functions in a similar manner to the TRH function test, is no longer conducted as TSH is no longer available on the market. |
|||
Alternative treatments should be prioritised in case of cardiac, electrolyte, and renal dyscrasias.<ref name="endo"/> |
|||
=== Imaging procedures === |
|||
Thyroid [[Medical ultrasound|sonography]], a diagnostic technique that has been utilized in human medicine for decades, has only recently been employed in veterinary medicine, primarily for research purposes. The primary reasons for this are the high equipment requirements and the associated high equipment costs. High-resolution linear transducers with at least 7.5 MHz, preferably 10 to 13 MHz, with a small contact surface are utilized.<ref>C. Poulsen Nautrup et al.: ''Schilddrüse und Nebenschilddrüsen.'' In: C. Poulsen Nautrup, R. Tobias (Ed.): ''Atlas und Lehrbuch der Ultraschalldiagnostik bei Hund und Katze.'' 2nd edition. Published by Schlütersche publishing company, Hannover 1998, {{ISBN|3-87706-663-1}}, p. 113–116.</ref> Sonography can be employed to diagnose thyroid enlargement in all hyperthyroid cats, whereas the diagnostic reliability of palpation is only 84%, even among experienced veterinarians.<ref>W. Kraft et al.: ''Symptome bei Hyperthyreose der Katze: eine retrospektive Studie.'' In: ''Kleintierpraxis.'' 44(10), 1999, p. 719–732. {{ISSN|0023-2076}}</ref> |
|||
Thyroid [[scintigraphy]] is a valuable diagnostic procedure, yet it is only available in a limited number of veterinary clinics. In this procedure, a cat is administered a [[radionuclide]] (e.g., the [[Iodine|iodine isotope]] 131I or the [[Isotopes of technetium|technetium isotope]] 99mTc), and its accumulation in the adenomas is then visualized. The principal advantage of this method is that the precise location of the tumors in the thyroid gland can be determined, which is advantageous with regard to surgical therapy. On rare occasions, additional thyroid tissue may colonize outside the thyroid gland ([[Ectopia (medicine)|ectopia]], particularly in the [[mediastinum]]) and become diseased as a result of disorders during [[organogenesis]]. Such displaced thyroid tissue can only be detected by scintigraphy.<ref>M. E. Peterson, D. V. Becker: ''Radionuclide thyroid imaging in 135 cats with hyperthyroidism.'' In: ''Vet. Radiol.'' 25(1), 1984, p. 23–27. {{ISSN|0196-3627}}</ref> |
|||
===Clinical examination=== |
|||
Multiple clinical signs that support a diagnosis can be observed in hyperthyroid cats during a clinical exam. The most useful symptoms for this is weight loss with concurrent polyphagia and [[tachycardia]].<ref name="endo"/> |
|||
Rates of [[hypocalcaemia]] following bilateral thyroidectomy range from 6–82% depending on the study and technique used. Mild hypocalcaemia often occurs as a result of an [[oedema]] and calcium depeletion. This mild hypocalcaemia typically resolves after several weeks. Severe hypocalcaemia is due to iatrogenic hypoparathyroidism and poses a long term risk to patients. Symptoms include: anorexia, restless behaviour, [[muscle spasms]], [[muscle tremors]], [[tetany]], and [[convulsion]]. [[Calcium]] and [[dihydrotachysterol]] can be used to treat hypocalcaemia with quick results. [[Ergocalciferol]] is a cheaper alternative that is sometimes used but is not recommended due to risk of [[hypercalcaemia]]<ref name="endo"/> |
|||
A [[goitre]] is swelling from an enlarged thyroid gland. This does not occur in all cases of hyperthyroidism, even those caused by adenoma; however it occurs in most cases and is a common way to support a hyperthyroid diagnosis caused by adenoma. Other conditions can cause a cervical mass that may resemble a goitre.<ref name="endo"/> |
|||
Following unilateral thyroidectomy thyroxine levels may fall into the hypothyroid reference range. Thyroid function will return within 1–3 months and thyroid hormone supplements are not required. Supplementation retards the growth of the thyroid tissue and will prevent the patient from reaching a euthyroid state.<ref name="endo"/> |
|||
===Differential diagnosis=== |
|||
Polyphagia and weight loss are two concurrent symptoms that has multiple causes, including: [[Diabetes in cats|diabetes mellitus]], [[malnutrition]], [[malabsorption]], and [[maldigestion]]. If these symptoms occur serum thyroxine levels should be tested to confirm or exclude a diagnosis of hyperthyroidism.<ref name="endo"/> |
|||
== Therapy == |
|||
Currently, three [[therapy]] options are available for hyperthyroidism in cats: the use of [[Antithyroid agent|thyrostatic]] drugs, [[Surgery|surgical]] removal of the diseased thyroid tissue, and radioiodine therapy. Regardless of the procedure selected, subsequent treatment of concomitant and secondary diseases (e.g., kidney damage, high blood pressure, heart disease) is typically necessary.<ref name=":6">Beate Egner, Anthony P. Carr: ''Hyperthyreose bei der Katze – Welche Rolle spielen ACE-Hemmer?'' In: ''kleintier konkret.'' 11, 2008, p. 11–14.</ref> In order to ascertain the potential adverse effects of reduced thyroid hormone levels on renal function, a 30-day course of medication is recommended prior to the implementation of more radical measures such as thyroidectomy or radioiodine therapy.<ref name=":2">Thomas Graves: ''Aktuelle Aspekte der Hyperthyreose bei der Katze.'' In: ''Vet. Focus.'' 19.3, 2009, p. 2–5.</ref> |
|||
Whilst iatrogenic hypothyroidism following a bilateral thyroidectomy is more common it does not always occur and should not be treated unless either clinical symptoms of hypothyroidism or signs of renal disease observed, or if it persists more than 3 months after the procedure.<ref name="endo"/> |
|||
=== Thyreostatics === |
|||
Therapy with thyrostatic agents is relatively straightforward and is therefore the most commonly used. Thyrostatic drugs inhibit the formation of thyroid hormones, but, in contrast to other methods, do not eliminate the pathologically altered tissue. Nevertheless, these drugs can usually be used in long-term therapy without any problems or can also be used to stabilize patients before a surgical procedure. In veterinary medicine, [[thiamazole]] (syn. methimazole, trade names Felimazole, Felidale and Thiamatab) or [[carbimazole]] (trade name Vidalta) are employed. Carbimazole is rapidly converted into methimazole when administered orally.<ref>M. E. Peterson, D. P. Aucoin: ''Comparison of disposition of carbimazole and methimazole in clinically normal cats.'' In: ''Res. Vet. Sci.'' 54(3), 1993, p. 351–355. {{PMID|8337482}}.</ref> According to the manufacturer, [[Adverse drug reaction|side effects]] (including vomiting, [[lethargy]], [[itch]]ing, [[Liver|liver disease]], and [[Complete blood count|blood count changes]]) occur in approximately 20% of cats, particularly with long-term treatment. However, these typically resolve once the drug is discontinued. Additionally, thiamazole cannot be used in cats with concomitant liver disease, [[Diabetes in cats|diabetes]], or [[Hemostasis|blood clotting disorders]]. |
|||
Thyroidectomy should be used for suspected carcinomata as it allows for histopathological examination of the tissue. Even when all visible neoplastic tissue is removed the carcinoma may reoccur, thus it is imperative to evaluate following the thyroidectomy. If carcinoma reoccurs radioactive iodine treatment should be undertaken.<ref name="endo"/> |
|||
[[Iopanoic acid]] may also be employed in the event of intolerance to thiamazole. It inhibits the conversion of T4 to T3 and has a negligible incidence of side effects.<ref name=":6" /> |
|||
=== |
===Anti-thyroid drugs=== |
||
[[File:Thioureylenes for cats.png|thumb|The chemical structure of the three main thioureylenes]] |
|||
Although surgical removal ([[thyroidectomy]]) is an effective treatment, it is also associated with a high risk of complications, particularly in cats with severe hyperthyroidism, due to the inherent risks associated with anesthesia. Prior to the surgical procedure, it is common practice to administer thyrostatic drugs. There are several techniques for the removal of the [[Parathyroid gland|thyroid gland]], with the objective of preserving the epithelial cells to the greatest extent possible. Additionally, there is a potential risk of injury to crucial cervical nerves ([[recurrent laryngeal nerve]], vagosympathetic trunk) during surgery. A total thyroidectomy results in a deficiency of thyroid hormones, which must be compensated for by lifelong administration.<ref>C. D. Welches et al.: ''Occurrence of Problems after Three Techniques of Bilateral Thyroidectomy in Cats.'' In: ''Vet. Surgery.'' 18(5), 1989, p. 392–396. {{PMID|2815557}}.</ref> In the event of unilateral removal, a transient hypothyroidism frequently develops postoperatively, although this is typically not a cause for concern.<ref name=":3" /> Furthermore, there is a risk of [[Relapse|recurrence]] with surgical removal, particularly in the presence of ectopic thyroid tissue.<ref>E. C. Naan et al.: ''Results of thyroidectomy in 101 cats with hyperthyroidism.'' In: ''Vet. Surg.'' 35(3), 2006, p. 287–293. {{PMID|16635010}}</ref> |
|||
[[Anti-thyroid drug]]s are a cheap option that does not require any form of hospital treatment as the medication can be given orally at home; however, the drugs must be given [[b.i.d.]], although this can be reduced to [[Quaque die|q.d.]] after a month, and success relies on owner compliance. Anti-thyroid drugs do not address the underlying thyroid issues and has several side effects such as [[Anorexia (medical condition)|anorexia]], [[emesis]], lethargy, [[thrombocytopaenia]], [[granulocytopaenia]], and [[hepatopathy]].<ref name="endo"/> |
|||
The three main anti-thyroid drugs, known as [[thioureylenes]], are [[methimazole]], [[carbimazole]], and [[propylthiouracil]]. Carbimazole is converted to methimazole by the body. These drugs concentrate within the thyroid gland and inhibit the ability of [[iodide]] and [[iodothyronines]] to form thyroxine and triiodothyronine which prevents thyroid synthesis. These drugs do not affect thyroid hormone that has already been produced.<ref name="endo"/> |
|||
=== Radioiodine therapy === |
|||
Radioiodine therapy is the treatment of choice due to its efficacy and tolerability. A single treatment is typically sufficient, eliminating the need for long-term drug treatment (which can be problematic in some cats) and the risks associated with surgical removal.<ref>M. E. Peterson: ''Radioiodine treatment of hyperthyroidism.'' In: ''Clin. Tech. Small Anim. Pract.'' 21(1), 2006, p. 34–39. {{PMID|16584029}}.</ref> However, it is associated with significant [[radiation protection]] requirements and is currently only available at two veterinary facilities in Germany. In addition to the limited availability, the associated costs and the need for hospitalization represent a disadvantage. In close consultation with the responsible supervisory authorities, it has been possible to reduce the required duration of hospitalization from approximately three weeks to a few days.<ref>M. Puille et al.: ''Radiojodtherapie bei Katzen: Strahlenschutz der Kontaktpersonen.'' In: ''Tierärztl Prax.'' 33 (K), 2005, p. 291–295. {{ISSN|1434-1239}}</ref> The necessary duration of hospitalization is determined by dosimetry and is seven to ten days.<ref name=":5">Andrea Monika Mathes, Reto Neiger: ''Hyperthyreose der Katze.'' In: ''Kleintierpraxis.'' 55, 2010, p. 685–698.</ref> |
|||
Of these drugs propylthiouracil is not recommended due to the common side effects including: anorexia, emesis, lethargy, [[immune-mediated haemolytic anaemia]], and [[thrombocytopaenia]].<ref name="endo"/> |
|||
== Thermal or chemical destruction of the thyroid gland == |
|||
Destruction of the thyroid tissue using a radiosurgical device under ultrasound control (thermal ablation) or by injection of 96% [[ethanol]] (chemical ablation) is practically no longer relevant. Both forms of treatment have increased side effects such as laryngeal paralysis or [[Horner's syndrome]].<ref name=":5" /> |
|||
Thioureylenes should not be used in patients with thyroid carcinoma these drugs increase the release of thyroid-stimulating hormone which aggravates growth of the tumour. The drugs themselves do not do anything to control the growth of tumours. The exception to this may be to stabilise symptoms before radioactive iodine treatment or thyroidectomy.<ref name="endo"/> |
|||
====Methimazole==== |
|||
Methimazole restores thyroxine levels to normal and resolves most hyperthyroid symptoms. This allows for the assessment of renal function before starting permanent treatment of the hyperthyroidism. Methimazole can also be used to stabilise patients with severe hyperthyroidism prior to surgery or radioactive iodine treatment. Methimazole is less effective in cats with more serious goitres and carcinomata.<ref name="endo"/> |
|||
Methimazole may also be applied [[topical]]ly to the [[Pinna (anatomy)|pinna]]e. Topical application reduces gastrointestinal side effects but can result in [[erythema]] and inflammation of the pinnae, this can be treated with [[glucocorticoid]]s. Topical methimazole is easier to apply for owners but more expensive than oral methimazole.<ref name="endo"/> |
|||
Between 10–25% of cats receiving methimazole will have mild side effects. These usually develop within the first 4–8 weeks and rarely develop beyond this. The most common side effects are anorexia, emesis, and lethargy. The side effects typically resolve either on their own or following a decrease in the dosage. Treatment is ceased in the event of gastrointestinal symptoms and not restarted until they have resolved. Gastrointestinal side effects are more common in cats receiving oral rather than topical treatment. Other mild side effects include haematological condition: [[eosinophilia]], [[leukopaenia]], and [[lymphocytosis]] occur at rates of 15% for oral and 5% for topical. These mild conditions do not require any cessation of methimazole.<ref name="endo"/> |
|||
Less common side effects include self-induced [[excoriation]] which occurs in 2–3% of cats receiving oral treatment and rarely in cats receiving topical treatment. Methimazole should be ceased in these cats and alternative treatments should be pursued for these cats. 3–9% of cats experience severe haematological reactions such as severe [[thrombocytopaenia]] alongside [[haemorrhage]], and [[neutropaenia]] alongside [[pyrexia]], anorexia, and infection. Any serious blood [[dyscrasia]] requires treatment to be immediately stopped and alternative treatments need to be considered. [[Myasthenia gravis]] is rare and has only been reported in cats treated orally. The understanding of it and its relation to methimazole is unknown but the [[immunomodulatory]] effects of the drug have been suspected as a cause.<ref name="endo"/> |
|||
Symptoms of [[Hypothyroidism in dogs#Cats|hypothyroidism]] are rare but cats with biochemical levels of thyroid hormone that falls into the hypothyroid range is common. [[Iatrogenic hypothyroidism]] presents an increased risk of [[azotaemia]] and any cat with iatrogenic hypothyroidism should have doses reduced and close monitoring following this.<ref name="endo"/> |
|||
====Carbimazole==== |
|||
Carbimazole is quickly transformed to methimazole after absorption or in the gastrointestinal tract. No studies compare the two but evidence suggests lower rates of adverse effects with no reports of severe blood disorders.<ref name="endo"/> |
|||
===Radioactive iodine=== |
|||
[[Radioactive iodine]] treatment results in quick decrease of thyroid hormone, does not require any anaesthetic or surgery, and in most cases is a single treatment. The disadvantages of radioactive iodine is the cost, the availability of it, and the local period a cat needs to stay in a specialised hospital as the radioactivity wears off. The success rate of radioactive iodine treatment is very high at around 95%.<ref name="endo"/> |
|||
Radioactive iodine is administered either [[intravenous]]ly or [[Subcutaenous injection|subcutaneous]]ly. The iodine gets transported to neoplastic and hyperplastic thyroid tissue. 40–90% of the iodine is excreted via defecation and urination. [[Gamma ray]]s and [[Beta particle|beta particles]] are emitted and these particles cause [[Thyroid follicular cell|follicular cell]]s to die off. The beta particles do not affect the [[parathyroid gland]] or any other part of the body due to their movement being limited to less than 2mm. The [[atrophied]] thyroid tissue will return to regular production of thyroid hormone which avoids the risk of iatrogenic hypothyroidism with other treatment options.<ref name="endo"/> In one study after 4 years 84% of cats were euthyroid and 4% were hypothyroid.<ref>{{cite journal | last=Théon | first=Alain P. | last2=Van Vechten | first2=Melinda K. | last3=Feldman | first3=Edward | title=Prospective randomized comparison of intravenous versus subcutaneous administration of radioiodine for treatment of hyperthyroidism in cats | journal=American Journal of Veterinary Research | volume=55 | issue=12 | date=1994-12-01 | issn=0002-9645 | doi=10.2460/ajvr.1994.55.12.1734 | pages=1734–1738}}</ref> |
|||
Thyroid carcinomata are uncommon but difficult to diagnose and require higher dosage to treat. [[Necrosis]] may occur following treatment if there is a lot of neoplastic tissue. This risk can be mitigated with surgery, although surgery itself carries risk. High radiation dosage often results in permanent hypothyroidism.<ref name="endo"/> |
|||
Thioureylenes induce radioresistance in humans which limits the effectiveness of radioactive iodine treatment. There is no evidence to support for this theory in felines; however, it is still recommended that anti-thyroid drugs should be discontinued for a week or two before starting treatment.<ref name="endo"/> |
|||
===Iodine intake=== |
|||
Limiting the iodine intake of a hyperthyroid cat is a cheap and easy way to manage hyperthyroidism and can be done by the owner with ease; however, it is not always effective, cats may refuse food or find other sources of iodine. The long term effects of this treatment are not currently known.<ref name="endo"/> |
|||
In one unpublished study 96% of cats being treated with a diet of less than 0.2mg/kg of iodine were euthyroid by 180 days. Most failures were attributed to cats having an alternative source of iodine. The long term effects of an iodine diet are not currently known. Iodine limited diets present an alternative for owners with financial issues and in cats with concurrent illness that would make other options more dangerous.<ref name="endo"/> |
|||
===Concurrent renal disease=== |
|||
Concurrent renal disease is not uncommon in hyperthyroid cats and renal disease can complicate treatment for hyperthyroidism. The two symptoms may mask the serum levels used to diagnose the other condition. Some estimates have up to 40% of hyperthyroid cats having [[chronic kidney disease]].<ref name="endo"/> |
|||
Because of the increased heart rate and decreased [[vascular resistance]] in hyperthyroid cats, hyperthyroid cats also have increased [[renal plasma flow]] and an increased [[glomerular filtration rate]]. When hyperthyroidism is treated these decrease and an increase of [[serum creatinine]] often occurs, which may cause [[azotaemia]] and in some cases [[renal failure]].<ref name="endo"/> In one study 15% of cats with treated hyperthyroidism developed [[azotaemia]].<ref>{{cite journal | last=Williams | first=T.L. | last2=Elliott | first2=J. | last3=Syme | first3=H.M. | title=Association of Iatrogenic Hypothyroidism with Azotemia and Reduced Survival Time in Cats Treated for Hyperthyroidism: Iatrogenic Hypothyroidism and Azotemia | journal=Journal of Veterinary Internal Medicine | volume=24 | issue=5 | date=2010 | doi=10.1111/j.1939-1676.2010.0566.x | pages=1086–1092}}</ref> |
|||
There are ways to try and measure if a cat will develop renal disease following hyperthyroidism but these do not accurately predict azotaemia and renal disease. Thus the best practice is to trial [[methimazole]] then measure serum biochemistry and analyse urine before starting permanent hyperthyroid treatment (radioactive iodine and thyroidectomy).<ref name="endo"/> |
|||
When cats have renal disease and hyperthyroidism dietary or drug related treatments are used to minimise hyperthyroid symptoms without hastening renal failure.<ref name="endo"/> |
|||
==Post-treatment== |
|||
Following treatment patients should receive a follow up after 1 month, 3 months, 6 months, and 12 months. A physical examination should be carried out alongside serum biochemistry and measurement of serum thyroxine levels to confirm the cat is no longer hyperthyroid and to diagnose iatrogenic hypothyroidism.<ref name="endo"/> |
|||
==Prognosis== |
|||
Between 93–95% of cats treated with radioactive iodine become euthyroid without further need for treatment. Reasons for failing to achieve euthyroidism can be due to adenoma, adenomatous hyperplasia, carcinoma, or an error in the administration of the treatment.<ref name="endo"/> |
|||
Rarely some cats will redevelop hyperthyroidism up to 6 years after treatment, this is likely due to new foci caused by new mutations in the tissue than any failure of treatment.<ref name="endo"/> |
|||
The average age of death for a cat that has received radioactive iodine treatment for hyperthyroidism is 15 years. Studies have found survival times of between 417 days and 2 years for all cats receiving any form of treatment.<ref name="endo"/> |
|||
== References == |
== References == |
||
{{reflist}} |
{{reflist}} |
||
== External links == |
|||
* [https://www.prokatze.de/hyperthyreose.htm Cat medicine: Feline hyperthyroidism] (with pictures) |
|||
[[Category:Cat diseases]] |
[[Category:Cat diseases]] |
||
[[Category:Endocrine |
[[Category:Endocrine diseases]] |
||
[[Category:Thyroid disease]] |
[[Category:Thyroid disease]] |
||
[[Category:Thyroid]] |
|||
Latest revision as of 02:49, 30 October 2024
| Hyperthyroidism | |
|---|---|
| Specialty | Endocrinology |
| Symptoms | Weight loss, polyphagia, polydipsia, tachycardia, polyuria, restlessness |
| Complications | Cardiac problems, cachexia, hypertension |
| Usual onset | Average 13 years |
| Duration | Lifelong if not properly treated |
| Causes | Tumour, with multifactorial cause behind it. |
| Risk factors | Age, diet, chemicals, non-Siamese breeds, indoor cats. |
| Diagnostic method | Measurement of serum thyroxine levels |
| Differential diagnosis | Diabetes mellitus |
| Prevention | Limiting iodine intake |
| Treatment | Thyroidectomy, radioactive iodine, anti-thyroid drugs |
| Medication | Methimazole, carbimazole |
| Prognosis | Average life expectancy of 15 years if treated |
| Frequency | Most common endocrinopathy in cats |
Feline hyperthyroidism is an endocrine disorder in which the thyroid gland produces too much thyroid hormone. Hyperthyroidism is the most common endocrinopathy of cats. The complete pathogenesis is not fully understood.
Background
[edit]In 1979 the first clinical report of a cat with hyperthyroidism was reported.[1] More studies and greater awareness would follow and today hyperthyroidism is a common condition in small animal practice. Whether that is due to increased prevalence or better testing is not entirely agreed upon.[2]
A study in 1987 transplanted thyroid tissue from affected cats into nude mice. The mice were administered levothyroxine, which suppresses thyroid-stimulating hormone. The thyroid cells remained in the hyperthyroid state. This study helped provide evidence for thyroid dysfunction as the cause rather than thyroid stimulation.[2]
Causes
[edit]The majority of cases of hyperthyroidism in cats are the result of benign tumours. The most commonly identified abnormalities of the thyroid gland in hyperthyroid cases are follicular cell adenoma and multinodular adenomatous hyperplasia. 1% to 3% of feline hyperthyroid cases are caused by malignant tumours. These are not always able to be distinguished from benign tumours.[2]
It is believed that the cause of hyperthyroidism is multifactorial. Studies have identified different mutations as causing thyroid cell autonomy.[2]
Iodine
[edit]Excessive iodine administration has been hypothesised as causing thyrotoxicosis in cats, as it does in humans; however, cats have been shown in multiple long terms studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. Iodide intake and its effects are unknown.[2]
Iodine amounts in cat food vary by as much as a factor of 30. Low, high, and variable iodine diets have been hypothesised as cause of feline hyperthyroidism.[2]
Soy isoflavones
[edit]Soy is often used in commercial cat food diets as a vegetable protein. In one study more than half of commercial cat foods surveyed contained soy isoflavones. Genistein and daidzein, both of which occur in soybeans, inhibit the enzymes thyroid peroxidase and thyroxine 5-deiodinase. This causes decreased thyroxine and triiodothyronine concentrations. In response to decreased triiodothyronine levels the body will produce more thyroid-stimulating hormone to normalise triiodothyronine levels, this has been shown to result in increased thyroxine levels. In addition these effects are heightened when a cat is suffering from iodine deficiency. However, further research is needed to confirm a link between soy isoflavones and hyperthyroidism.[2]
Goitrogens
[edit]Goitrogens are chemicals that disrupt thyroid hormone production. Chemicals such as bisphenol A and polybrominated diphenyl ethers have been shown to exist in indoor cats at heightened levels and are hypothesised to be a potential cause of hyperthyroidism. Bisphenol A is common in canned cat foods, although no association between it and hyperthyroidism has currently been demonstrated. Polybrominated diphenyl ethers (PBDEs) are often used as fire retardants and are known to be a goitrogen; however, levels of these chemicals have been shown to be the same in euthyroid cats as hyperthyroid cats. No association has been proven in a study for the association between PBDEs and hyperthyroidism.[2]
Signs and symptoms
[edit]Hyperthyroidism is a disease that slowly progresses and symptoms are very subtle at first. It can take up to two years from onset of symptoms for a diagnosis of hyperthyroidism to be made.[2]
The most common symptom is weight loss, occurring in 98% of hyperthyroid cases. Other symptoms that occur more than 50% of the time, include: a palpable thyroid gland (91%); polyphagia (81%), without obesity; restlessness (76%); tachycardia (66%); polydipsia and polyuria (60%); emesis; and cardiac murmur (53%).[2]
Less common symptoms include: diarrhoea (33%), increased volume of faeces (31%), anorexia (26%), polypnoea (25%), myasthenia (25%), muscle tremors (18%), congestive heart failure (12%), excessive nail growth (12%), and dyspnoea (11%).[2]
Systolic hypertension is reported in 10-15% of cats that have been diagnosed with hyperthyroidism. Some normotensive hyperthyroid cats develop hypertension after having their hyperthyroidism treated. The reasons for this are not currently known.[2]
Cachexia is a possible symptom in hyperthyroid cats but due to improved screening and diagnostics it is less common for a cat to become cachectic before starting treatment.[2]
Ventroflexion of the head is a rare symptom observed in hyperthyroid cats but the last case report is from 1994. Due to the rarity the pathogenesis behind this is not known.[2]
Cardiac conditions have become less common over the years in hyperthyroid cats due to earlier diagnosis and improved treatment. The most common cardiovascular issues are tachyarrhythmias, cardiac murmurs, and gallop rhythms. The cause of these conditions is due to how triiodothyronine increases cardiac output.[2]
Thyroid cysts can occur in cases of adenoma, adenomatous hyperplasia, or carcinoma. These cysts have a high concentration of thyroxine. Cysts should not be treated with radioactive iodine and instead be surgically removed.[2]
Symptoms of hyperadrenocorticism can occur in hyperthyroid cats. Adrenocorticotropic hormone levels are raised in some hyperthyroid cats and some studies have documented acromegaly in hyperthyroid cats.[2]
Risk factors
[edit]Multiple case control studies have looked at diet and hyperthyroidism. These studies have found associations between commercial diets and hyperthyroidism. Other risk factors identified include non-Siamese related breeds, lack of outdoor access, flea medication, pesticides, certain cat litters, female sex, sleeping on the floor, organic fertiliser, human baby food, carpet cleaners, natural gas, lack of deworming, and a fish diet.[3] These mixed results suggest a multifactoral cause with diet being having an important role.[2]
Age of onset
[edit]The age of onset has been reported to be between 4–22 years with an average of 13 years, with some individual cases occurring in cats below the age of 4 years. More than 95% of cases occur in cats 8 years and older.[2]
Diagnosis
[edit]Testing for hyperthyroidism is routine for elderly cats and standard blood tests that can be performed in clinic allow for measurement of serum thyroxine levels. With this advancement diagnosis is often made before clinical signs are well noticeable and severe.[2]
Diagnosing hyperthyroid cats is not just to confirm the presence of the condition but also what symptoms are present. For example more serious cardiac conditions require special treatment compared to a cat without any serious cardiac abnormalities. Cats with hyperthyroidism should have a complete blood count test, a biochemistry profile of blood serum, urinalysis, measurement of thyroxine serum levels, thoracic radiography, and measurement of blood pressure. If heart disease is suspected electrocardiography and echocardiography should be performed.[2]
Measurement serum thyroxine levels is the best method of confirming hyperthyroidism. Measurement of serum concentrations triiodothyronine are not useful for diagnosing hyperthyroidism and neither is measurement of serum thyroid-stimulating hormone levels. Thyroxine levels on their own should not be used to diagnose hyperthyroidism: if a patient is displaying no symptoms of hyperthyroidism but test results show heightened serum thyroxine levels the possibility of other causes such as an error in the test have to be considered and excluded before establishing a diagnosis.[2]
Complete blood count
[edit]Common findings of a complete blood count on a hyperthyroid cat include a small increase in packed cell volume (40-50%), macrocytosis (20%), and Heinz bodies. Anaemia is rare. Increased erythrocyte count is due to thyroid hormone stimulating erythropoietin secretion. Leukocytes and thrombocytes usually have normal results, some changes may occur but these are not specific and are not consistent.[2]
Serum chemistry
[edit]The most commonly noticed change in serum chemistry is a non-major increase in liver enzymes. More than 90% of hyperthyroid cats have an increase in either serum concentration of alanine aminotransferase and alkaline phosphatase, with more than 75% of cats having an increase in both. These increases are small and typically below 500 IU/L. Higher increases may be due to hepatic disease, although many other conditions can cause an increase in liver enzymes.[2]
Fructosamine concentrations are lowered by serum protein metabolism, which is increased in hyperthyroid cats. Fructosamine concentrations are significantly lower in hyperthyroid cats, with half of hyperthyroid cats having levels outside the reference range. Following treatment for hyperthyroidism these concentrations rise to normal levels. Diabetes mellitus also affects fructosamine concentration and this should not be relied on in diabetic cats. Hyperglycaemia can occur in hyperthyroid cats independent of diabetes mellitus. Frucotsamine measurement cannot reliably differentiate non-diabetes related hyperglycaemia.[2]
Blood urea and creatinine
[edit]In roughly 10% of hyperthyroid cats serum creatinine levels are increased to the point of azotaemia. In 10-20% of cats increased serum concentration of blood urea nitrogen is observed. Although the prevalence of hyperthyroid cats with chronic kidney disease is higher than this due to how hyperthyroidism results in an increase to the glomerular filtration rate. Most cats with chronic kidney disease and hyperthyroidism have the kidney disease go undiagnosed until after treatment for hyperthyroidism has begun. Between 15% and 49% of non-azotaemic hyperthyroid cats will develop azotaemia following treatment for hyperthyroidism. Further complicating the diagnosis is the shared symptoms between the conditions. It is not currently known whether the effects of hyperthyroidism cause or worsen renal disease. Chronic kidney disease is not an uncommon condition in elderly cats further complicating determination of a relationship between the two conditions.[2]
Urinalysis
[edit]Common abnormalities found during urinalysis of hyperthyroid cats include: decreased urine specific gravity, proteinuria, symptoms of urinary tract infections, and ketonuria.[2]
Decreases to urine specific gravity are likely not the result of hyperthyroidism but instead due to chronic kidney disease.[2] In a 2009 study most cats that had been treated for hyperthyroidism with radioactive iodine did not have changes to urine specific gravity levels.[4]
Proteinuria is reported in 75–80% of cats with hyperthyroidism. Likely causes of this are glomerular hyperfiltration and glomerular hypertension, and structural changes in the glomerular filtration barrier.[2]
Radiography
[edit]Thoracic radiographs do not help diagnose hyperthyroidism, instead they help diagnose concurrent illnesses like thoracic neoplasia. If respiratory distress, tachypnoea, tachycardia, arrhythmia, or heart murmur are observed then thoracic radiographs should be taken. Congestive heart failure is uncommon. The most common finding is cardiomegaly.[2]
Echocardiography
[edit]The most common echocardiographic abnormalities in hyperthyroid cats is hypertrophy of the left ventricular caudal wall and interventricular septum. The effects of thyroid hormone are known and explain most cardiac abnormalities;[2] however, one study found only 37% of hyperthyroid cats to have an echocardiographic abnormality, with 32% having abnormalities following treatment.[5]
Clinical examination
[edit]Multiple clinical signs that support a diagnosis can be observed in hyperthyroid cats during a clinical exam. The most useful symptoms for this is weight loss with concurrent polyphagia and tachycardia.[2]
A goitre is swelling from an enlarged thyroid gland. This does not occur in all cases of hyperthyroidism, even those caused by adenoma; however it occurs in most cases and is a common way to support a hyperthyroid diagnosis caused by adenoma. Other conditions can cause a cervical mass that may resemble a goitre.[2]
Thyrotropin-releasing hormone stimulation
[edit]Following a test for blood serum thyroxine levels thyrotropin-releasing hormone can be administered to cats intravenously and after 4 hours another blood test can be taken to measure thyroxine serum levels. If the thyroxine levels have increased this indicates the cat is euthyroid but if levels remain stagnant it suggests hyperthyroidism. This test is rarely performed due to side effects. The most common side effects being: emesis, tachypnoea, and defecation.[2]
Scintigraphy
[edit]Scintigraphy helps to show information on the thyroid gland and tumours affecting it. It is useful for identifying the size of the tumour, whether it is unilateral or bilateral, and how much radiation is required for treatment. Iodine-131, Iodine-123, and technetium-99m pertechnetate are all used for this. Pertechnetate is the most commonly used due its low cost, quick uptake, and short half-life. Scintigraphy can also be used to confirm hyperthyroidism when clinical symptoms are present but serum thyroxine levels remain within reference. Drugs such as methimazole and anaesthetics can influence results if not discontinued before scintigraphy.[2]
Ultrasonography
[edit]Ultrasonography is an alternative option to scintigraphy. Ultrasounds provide less information than scintigraphy but is easier and cheaper to conduct. Ultrasonography allows for the state of the thyroid gland to be evaluated and for an estimation of the volume but does not allow for evaluation of tissue.[2]
Differential diagnosis
[edit]Polyphagia and weight loss are two concurrent symptoms that have multiple causes, including: diabetes mellitus, malnutrition, malabsorption, and maldigestion. If these symptoms occur serum thyroxine levels should be tested to confirm or exclude a diagnosis of hyperthyroidism.[2]
Carcinoma
[edit]Thyroid carcinoma has largely the same symptoms as benign tumours. Hypercalcaemia may be seen in a patient with carcinoma. Symptoms that may be identified via radiography include: cardiomegaly, signs of congestive heart failure, signs of pulmonary metastasis, and mediastinal masses. The only way to definitively diagnose a thyroid carcinoma is via histopathological examination of the affected tissue. If treatment has failed thyroid carcinoma should be considered as a possibility.[2]
Treatment
[edit]Hyperthyroidism can be either managed with anti-thyroid drugs or restricting iodine intake, or it can be treated with radioactive iodine or thyroidectomy for a permanent solution. Choice of treatment depends on a multitude of factors including cost, availability of the treatments, and age and health of the patient. In areas where available, radioactive iodine treatment is the preferred method of treating hyperthyroidism due to the low risks and high efficacy.[2]
Surgery
[edit]Surgery is a cheaper option than radioactive iodine treatment and does not require any equipment not found in a standard veterinary clinic. It also results in a quicker decrease in thyroid hormone levels. Surgery however can be more risky to elderly cats given the increased risk of anaesthetic complication. A bad surgery can result in nerve damage or fail to fully remove all the abnormal tissue. It may also result in iatrogenic hypothyroidism and hypoparathyroidism.[2]
Surgery has become a less common treatment option with improvement of access to radioactive iodine treatment and due to risks. Risks include: ectopic tissue being leftover, hypothyroidism, hypoparathyroidism. Less often laryngeal nerve damage and Horner's syndrome may occur. These risks can be prevented with a more experienced surgeon and proper imaging of the thyroid gland before removal.[2]
Thyroidectomy allows for shorter hospitalisation time and for histopathological evaluation of neoplastic tissue. Thyroidectomy is not recommended when the disease is bilateral.[2]
Alternative treatments should be prioritised in case of cardiac, electrolyte, and renal dyscrasias.[2]
Rates of hypocalcaemia following bilateral thyroidectomy range from 6–82% depending on the study and technique used. Mild hypocalcaemia often occurs as a result of an oedema and calcium depeletion. This mild hypocalcaemia typically resolves after several weeks. Severe hypocalcaemia is due to iatrogenic hypoparathyroidism and poses a long term risk to patients. Symptoms include: anorexia, restless behaviour, muscle spasms, muscle tremors, tetany, and convulsion. Calcium and dihydrotachysterol can be used to treat hypocalcaemia with quick results. Ergocalciferol is a cheaper alternative that is sometimes used but is not recommended due to risk of hypercalcaemia[2]
Following unilateral thyroidectomy thyroxine levels may fall into the hypothyroid reference range. Thyroid function will return within 1–3 months and thyroid hormone supplements are not required. Supplementation retards the growth of the thyroid tissue and will prevent the patient from reaching a euthyroid state.[2]
Whilst iatrogenic hypothyroidism following a bilateral thyroidectomy is more common it does not always occur and should not be treated unless either clinical symptoms of hypothyroidism or signs of renal disease observed, or if it persists more than 3 months after the procedure.[2]
Thyroidectomy should be used for suspected carcinomata as it allows for histopathological examination of the tissue. Even when all visible neoplastic tissue is removed the carcinoma may reoccur, thus it is imperative to evaluate following the thyroidectomy. If carcinoma reoccurs radioactive iodine treatment should be undertaken.[2]
Anti-thyroid drugs
[edit]
Anti-thyroid drugs are a cheap option that does not require any form of hospital treatment as the medication can be given orally at home; however, the drugs must be given b.i.d., although this can be reduced to q.d. after a month, and success relies on owner compliance. Anti-thyroid drugs do not address the underlying thyroid issues and has several side effects such as anorexia, emesis, lethargy, thrombocytopaenia, granulocytopaenia, and hepatopathy.[2]
The three main anti-thyroid drugs, known as thioureylenes, are methimazole, carbimazole, and propylthiouracil. Carbimazole is converted to methimazole by the body. These drugs concentrate within the thyroid gland and inhibit the ability of iodide and iodothyronines to form thyroxine and triiodothyronine which prevents thyroid synthesis. These drugs do not affect thyroid hormone that has already been produced.[2]
Of these drugs propylthiouracil is not recommended due to the common side effects including: anorexia, emesis, lethargy, immune-mediated haemolytic anaemia, and thrombocytopaenia.[2]
Thioureylenes should not be used in patients with thyroid carcinoma these drugs increase the release of thyroid-stimulating hormone which aggravates growth of the tumour. The drugs themselves do not do anything to control the growth of tumours. The exception to this may be to stabilise symptoms before radioactive iodine treatment or thyroidectomy.[2]
Methimazole
[edit]Methimazole restores thyroxine levels to normal and resolves most hyperthyroid symptoms. This allows for the assessment of renal function before starting permanent treatment of the hyperthyroidism. Methimazole can also be used to stabilise patients with severe hyperthyroidism prior to surgery or radioactive iodine treatment. Methimazole is less effective in cats with more serious goitres and carcinomata.[2]
Methimazole may also be applied topically to the pinnae. Topical application reduces gastrointestinal side effects but can result in erythema and inflammation of the pinnae, this can be treated with glucocorticoids. Topical methimazole is easier to apply for owners but more expensive than oral methimazole.[2]
Between 10–25% of cats receiving methimazole will have mild side effects. These usually develop within the first 4–8 weeks and rarely develop beyond this. The most common side effects are anorexia, emesis, and lethargy. The side effects typically resolve either on their own or following a decrease in the dosage. Treatment is ceased in the event of gastrointestinal symptoms and not restarted until they have resolved. Gastrointestinal side effects are more common in cats receiving oral rather than topical treatment. Other mild side effects include haematological condition: eosinophilia, leukopaenia, and lymphocytosis occur at rates of 15% for oral and 5% for topical. These mild conditions do not require any cessation of methimazole.[2]
Less common side effects include self-induced excoriation which occurs in 2–3% of cats receiving oral treatment and rarely in cats receiving topical treatment. Methimazole should be ceased in these cats and alternative treatments should be pursued for these cats. 3–9% of cats experience severe haematological reactions such as severe thrombocytopaenia alongside haemorrhage, and neutropaenia alongside pyrexia, anorexia, and infection. Any serious blood dyscrasia requires treatment to be immediately stopped and alternative treatments need to be considered. Myasthenia gravis is rare and has only been reported in cats treated orally. The understanding of it and its relation to methimazole is unknown but the immunomodulatory effects of the drug have been suspected as a cause.[2]
Symptoms of hypothyroidism are rare but cats with biochemical levels of thyroid hormone that falls into the hypothyroid range is common. Iatrogenic hypothyroidism presents an increased risk of azotaemia and any cat with iatrogenic hypothyroidism should have doses reduced and close monitoring following this.[2]
Carbimazole
[edit]Carbimazole is quickly transformed to methimazole after absorption or in the gastrointestinal tract. No studies compare the two but evidence suggests lower rates of adverse effects with no reports of severe blood disorders.[2]
Radioactive iodine
[edit]Radioactive iodine treatment results in quick decrease of thyroid hormone, does not require any anaesthetic or surgery, and in most cases is a single treatment. The disadvantages of radioactive iodine is the cost, the availability of it, and the local period a cat needs to stay in a specialised hospital as the radioactivity wears off. The success rate of radioactive iodine treatment is very high at around 95%.[2]
Radioactive iodine is administered either intravenously or subcutaneously. The iodine gets transported to neoplastic and hyperplastic thyroid tissue. 40–90% of the iodine is excreted via defecation and urination. Gamma rays and beta particles are emitted and these particles cause follicular cells to die off. The beta particles do not affect the parathyroid gland or any other part of the body due to their movement being limited to less than 2mm. The atrophied thyroid tissue will return to regular production of thyroid hormone which avoids the risk of iatrogenic hypothyroidism with other treatment options.[2] In one study after 4 years 84% of cats were euthyroid and 4% were hypothyroid.[6]
Thyroid carcinomata are uncommon but difficult to diagnose and require higher dosage to treat. Necrosis may occur following treatment if there is a lot of neoplastic tissue. This risk can be mitigated with surgery, although surgery itself carries risk. High radiation dosage often results in permanent hypothyroidism.[2]
Thioureylenes induce radioresistance in humans which limits the effectiveness of radioactive iodine treatment. There is no evidence to support for this theory in felines; however, it is still recommended that anti-thyroid drugs should be discontinued for a week or two before starting treatment.[2]
Iodine intake
[edit]Limiting the iodine intake of a hyperthyroid cat is a cheap and easy way to manage hyperthyroidism and can be done by the owner with ease; however, it is not always effective, cats may refuse food or find other sources of iodine. The long term effects of this treatment are not currently known.[2]
In one unpublished study 96% of cats being treated with a diet of less than 0.2mg/kg of iodine were euthyroid by 180 days. Most failures were attributed to cats having an alternative source of iodine. The long term effects of an iodine diet are not currently known. Iodine limited diets present an alternative for owners with financial issues and in cats with concurrent illness that would make other options more dangerous.[2]
Concurrent renal disease
[edit]Concurrent renal disease is not uncommon in hyperthyroid cats and renal disease can complicate treatment for hyperthyroidism. The two symptoms may mask the serum levels used to diagnose the other condition. Some estimates have up to 40% of hyperthyroid cats having chronic kidney disease.[2]
Because of the increased heart rate and decreased vascular resistance in hyperthyroid cats, hyperthyroid cats also have increased renal plasma flow and an increased glomerular filtration rate. When hyperthyroidism is treated these decrease and an increase of serum creatinine often occurs, which may cause azotaemia and in some cases renal failure.[2] In one study 15% of cats with treated hyperthyroidism developed azotaemia.[7]
There are ways to try and measure if a cat will develop renal disease following hyperthyroidism but these do not accurately predict azotaemia and renal disease. Thus the best practice is to trial methimazole then measure serum biochemistry and analyse urine before starting permanent hyperthyroid treatment (radioactive iodine and thyroidectomy).[2]
When cats have renal disease and hyperthyroidism dietary or drug related treatments are used to minimise hyperthyroid symptoms without hastening renal failure.[2]
Post-treatment
[edit]Following treatment patients should receive a follow up after 1 month, 3 months, 6 months, and 12 months. A physical examination should be carried out alongside serum biochemistry and measurement of serum thyroxine levels to confirm the cat is no longer hyperthyroid and to diagnose iatrogenic hypothyroidism.[2]
Prognosis
[edit]Between 93–95% of cats treated with radioactive iodine become euthyroid without further need for treatment. Reasons for failing to achieve euthyroidism can be due to adenoma, adenomatous hyperplasia, carcinoma, or an error in the administration of the treatment.[2]
Rarely some cats will redevelop hyperthyroidism up to 6 years after treatment, this is likely due to new foci caused by new mutations in the tissue than any failure of treatment.[2]
The average age of death for a cat that has received radioactive iodine treatment for hyperthyroidism is 15 years. Studies have found survival times of between 417 days and 2 years for all cats receiving any form of treatment.[2]
References
[edit]- ^ M. E. Peterson et al.: Spontaneous hyperthyroidism in the cat. In: Proc. Am. College Vet. Intern. Med. 1979, p. 108.
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs Scott-Moncrieff, J. Catherine (2015). "Feline Hyperthyroidism". In Feldman, Edward C.; Nelson, Richard W.; Reusch, Claudia; Scott-Moncrieff, J. Catharine (eds.). Canine and feline endocrinology (Fourth ed.). St. Louis, Missouri: Elsevier Saunders. pp. 137–190. ISBN 978-1-4557-4456-5.
- ^ Edinboro, Charlotte H; Scott-Moncrieff, J Catharine; Glickman, Larry T (2010). "Feline Hyperthyroidism: Potential Relationship with Iodine Supplement Requirements of Commercial Cat Foods". Journal of Feline Medicine and Surgery. 12 (9): 672–679. doi:10.1016/j.jfms.2010.07.011. ISSN 1098-612X. PMC 11149000.
- ^ van Hoek, I.; Lefebvre, H.P.; Peremans, K.; Meyer, E.; Croubels, S.; Vandermeulen, E.; Kooistra, H.; Saunders, J.H.; Binst, D.; Daminet, S. (2009). "Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine". Domestic Animal Endocrinology. 36 (1). Elsevier BV: 45–56. doi:10.1016/j.domaniend.2008.10.001. ISSN 0739-7240.
- ^ Weichselbaum, Ralph C.; Feeney, Daniel A.; Jessen, Carl R. (2005). "RELATIONSHIP BETWEEN SELECTED ECHOCARDIOGRAPHIC VARIABLES BEFORE AND AFTER RADIOIODINE TREATMENT IN 91 HYPERTHYROID CATS". Veterinary Radiology & Ultrasound. 46 (6): 506–513. doi:10.1111/j.1740-8261.2005.00099.x. ISSN 1058-8183.
- ^ Théon, Alain P.; Van Vechten, Melinda K.; Feldman, Edward (1994-12-01). "Prospective randomized comparison of intravenous versus subcutaneous administration of radioiodine for treatment of hyperthyroidism in cats". American Journal of Veterinary Research. 55 (12): 1734–1738. doi:10.2460/ajvr.1994.55.12.1734. ISSN 0002-9645.
- ^ Williams, T.L.; Elliott, J.; Syme, H.M. (2010). "Association of Iatrogenic Hypothyroidism with Azotemia and Reduced Survival Time in Cats Treated for Hyperthyroidism: Iatrogenic Hypothyroidism and Azotemia". Journal of Veterinary Internal Medicine. 24 (5): 1086–1092. doi:10.1111/j.1939-1676.2010.0566.x.