Title: Feline hyperthyroidism

{{Short description|Endocrine disorder in cats}}
{{Infobox medical condition
 |name            = Hyperthyroidism
 |synonyms        = &lt;!-- or |synonym= --&gt;
 |image           = 41598 2024 79564 Fig1 HTML.webp
 |image_upright   = 
 |image_size      = 
 |alt             = 
 |image_thumbtime = 
 |caption         = Two hyperthyroid cats
 |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     = &lt;!-- or |eponym= --&gt;
}}

'''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 ==
In 1979 the first clinical report of a cat with hyperthyroidism was reported.&lt;ref&gt;M. E. Peterson et al.: ''Spontaneous hyperthyroidism in the cat.'' In: ''Proc. Am. College Vet. Intern. Med.'' 1979, p. 108.&lt;/ref&gt; 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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

== Causes ==
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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;&gt;{{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}}&lt;/ref&gt;

===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-term studies to be able to regulate their levels of thyroid hormone within safe ranges when administered iodine. [[Iodide]] intake and its effects are unknown.&lt;ref name=&quot;endo&quot;/&gt;

Iodine amounts in cat food vary by as much as a factor of 30. Low, high, and variable iodine diets have been hypothesised as a cause of feline hyperthyroidism.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

== Signs and symptoms ==
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.&lt;ref name=&quot;endo&quot;/&gt;

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%).&lt;ref name=&quot;endo&quot;/&gt;

Less common symptoms include: [[diarrhoea]] (33%), increased volume of [[faeces]] (31%), [[Anorexia (symptom)|anorexia]] (26%), [[polypnoea]] (25%), [[myasthenia]] (25%), [[muscle tremor]]s (18%), [[congestive heart failure]] (12%), excessive nail growth (12%), and [[dyspnoea]] (11%).&lt;ref name=&quot;endo&quot;/&gt;

[[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.&lt;ref name=&quot;endo&quot;/&gt;

[[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.&lt;ref name=&quot;endo&quot;/&gt;

[[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.&lt;ref name=&quot;endo&quot;/&gt;

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]].&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref&gt;{{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 }}&lt;/ref&gt; These mixed results suggest a multifactoral cause with diet being having an important role.&lt;ref name=&quot;endo&quot;/&gt;

===Age of onset===
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.&lt;ref name=&quot;endo&quot;/&gt;

== Diagnosis ==
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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

=== Complete blood count ===
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. [[Leukocyte]]s and [[thrombocyte]]s usually have normal results, some changes may occur but these are not specific and are not consistent.&lt;ref name=&quot;endo&quot;/&gt;

=== Serum chemistry ===
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 [[International unit|IU/L]]. Higher increases may be due to [[hepatic disease]], although many other conditions can cause an increase in liver enzymes.&lt;ref name=&quot;endo&quot;/&gt;

[[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 in cats|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.&lt;ref name=&quot;endo&quot;/&gt;

===Blood urea and creatinine===
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.&lt;ref name=&quot;endo&quot;/&gt;

===Urinalysis===
Common abnormalities found during [[urinalysis]] of hyperthyroid cats include: decreased [[urine specific gravity]], [[proteinuria]], symptoms of [[urinary tract infection]]s, and [[ketonuria]].&lt;ref name=&quot;endo&quot;/&gt;

Decreases to urine specific gravity are likely not the result of hyperthyroidism but instead due to chronic kidney disease.&lt;ref name=&quot;endo&quot;/&gt; In a 2009 study most cats that had been treated for hyperthyroidism with [[radioactive iodine]] did not have changes to urine specific gravity levels.&lt;ref&gt;{{cite journal | last=van Hoek | first=I. | last2=Lefebvre | first2=H.P. | last3=Peremans | first3=K. | last4=Meyer | first4=E. | last5=Croubels | first5=S. | last6=Vandermeulen | first6=E. | last7=Kooistra | first7=H. | last8=Saunders | first8=J.H. | last9=Binst | first9=D. | last10=Daminet | first10=S. | title=Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine | journal=Domestic Animal Endocrinology | publisher=Elsevier BV | volume=36 | issue=1 | year=2009 | issn=0739-7240 | doi=10.1016/j.domaniend.2008.10.001 | pages=45–56}}&lt;/ref&gt;

[[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.&lt;ref name=&quot;endo&quot;/&gt;

===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]].&lt;ref name=&quot;endo&quot;/&gt;

===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;&lt;ref name=&quot;endo&quot;/&gt; however, one study found only 37% of hyperthyroid cats to have an echocardiographic abnormality, with 32% having abnormalities following treatment.&lt;ref&gt;{{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 &amp; Ultrasound | volume=46 | issue=6 | date=2005 | issn=1058-8183 | doi=10.1111/j.1740-8261.2005.00099.x | pages=506–513}}&lt;/ref&gt;

===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]].&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

===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 [[defaecation]].&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

==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.&lt;ref name=&quot;endo&quot;/&gt;

===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 failure to fully remove all the abnormal tissue. It may also result in [[iatrogenic]] [[Hypothyroidism in dogs#Cats|hypothyroidism]] and [[hypoparathyroidism]].&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

Thyroidectomy allows for shorter hospitalisation time and for histopathological evaluation of neoplastic tissue. Thyroidectomy is not recommended when the disease is bilateral.&lt;ref name=&quot;endo&quot;/&gt;

Alternative treatments should be prioritised in case of cardiac, electrolyte, and renal dyscrasias.&lt;ref name=&quot;endo&quot;/&gt;

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 depletion. 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]]&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

===Anti-thyroid drugs===
[[File:Thioureylenes for cats.png|thumb|The chemical structure of the three main thioureylenes]]
[[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]].&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

Of these drugs propylthiouracil is not recommended due to the common side effects including anorexia, emesis, lethargy, [[immune-mediated haemolytic anaemia]], and [[thrombocytopaenia]].&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

====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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

Between 10 and 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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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. [[Hypothyroidism_in_dogs#Iatrogenic|Iatrogenic hypothyroidism]] presents an increased risk of [[azotaemia]] and any cat with iatrogenic hypothyroidism should have doses reduced and close monitoring following this.&lt;ref name=&quot;endo&quot;/&gt;

====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.&lt;ref name=&quot;endo&quot;/&gt;

===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%.&lt;ref name=&quot;endo&quot;/&gt;

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]]s 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.&lt;ref name=&quot;endo&quot;/&gt; In one study after 4 years 84% of cats were euthyroid and 4% were hypothyroid.&lt;ref&gt;{{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}}&lt;/ref&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

===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.&lt;ref name=&quot;endo&quot;/&gt;

In one unpublished study 96% of cats being treated with a diet of less than 0.2&amp;nbsp;mg/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.&lt;ref name=&quot;endo&quot;/&gt;

===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]].&lt;ref name=&quot;endo&quot;/&gt;

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]].&lt;ref name=&quot;endo&quot;/&gt; In one study 15% of cats with treated hyperthyroidism developed [[azotaemia]].&lt;ref&gt;{{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| doi-access=free }}&lt;/ref&gt;

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).&lt;ref name=&quot;endo&quot;/&gt;

When cats have renal disease and hyperthyroidism dietary or drug related treatments are used to minimise hyperthyroid symptoms without hastening renal failure.&lt;ref name=&quot;endo&quot;/&gt;

==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.&lt;ref name=&quot;endo&quot;/&gt;

==Prognosis==
Between 93 and 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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

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.&lt;ref name=&quot;endo&quot;/&gt;

== References ==
{{reflist}}

[[Category:Cat diseases]]
[[Category:Endocrine diseases]]
[[Category:Thyroid disease]]