Difference between revisions of "Hyperthyroidism"

From Cat
 
(53 intermediate revisions by the same user not shown)
Line 1: Line 1:
[[Image:hyper1.jpg|thumb|Abyssinian with hyperthyroidism]]
+
__NOTOC__
 +
[[Image:hyper1.jpg|thumb|Cachexia in an [[Abyssinian]] associated with hyperthyroidism]]
 
[[Image:thyroid4.jpg|thumb|Feline hyperthyroidism, thyroid nodule]]
 
[[Image:thyroid4.jpg|thumb|Feline hyperthyroidism, thyroid nodule]]
[[Image:thy01.jpg|thumb|Enlarged thytroid gland in a DSH cat]]
+
[[Image:thy01.jpg|thumb|Enlarged thyroid gland in a [[Domestic shorthair]] cat]]
 
[[Image:Thy02.jpg|thumb|Thyroidectomy being performed on above cat]]
 
[[Image:Thy02.jpg|thumb|Thyroidectomy being performed on above cat]]
 
[[Image:hyper2.jpg|thumb]]
 
[[Image:hyper2.jpg|thumb]]
 
[[Image:hyper3.jpg|thumb|Lateral and ventrodorsal radiographs of a cat with hyperthyroidism and hyperthyroid heart disease. Notice the enlarged cardiac silhouette]]
 
[[Image:hyper3.jpg|thumb|Lateral and ventrodorsal radiographs of a cat with hyperthyroidism and hyperthyroid heart disease. Notice the enlarged cardiac silhouette]]
 +
Hyperthyroidism is the most common endocrine disorder affecting the [[thyroid gland]] of older cats, characterized classically by elevated total thyroxine (T4) above the reference range of 20-40 nmol/L.
  
Hyperthyroidism is the most common endocrine disorder affecting older cats. It is a disease of the thyroid gland, which lies as two small glands in the neck region beside the trachea. It is uncommon to see hyperthyroidism in cats under 10 years of age.
+
[[Iodine]] content of the diet has been suggested as a likely etiologic factor in feline hyperthyroidism<ref>Mumma, RO ''et al'' (1986) Toxic and protective constituents in pet foods. ''Am J Vet Res'' '''47''':1633-1637</ref>.
  
Feline hyperthyroidism is mainly caused by benign nodular hyperplasia, adenomatous hyperplasia or a functional thyroid adenoma involving one or both sides of the thyroid gland and inevitably leading to an increase in the size of the gland. Certain factors may contribute, since it is seen in cats;
+
It is uncommon to see hyperthyroidism in cats under 10 years of age, and is commonly caused by a benign thyroid adenoma or adenomatous hyperplasia present in one or both lobes of the thyroid gland. Fewer than 2% of cases occur as a result of a functional thyroid carcinoma<ref>Martin, KM ''et al'' (2000) Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. ''J Am Vet Med Assoc'' '''217''':853-856</ref>.
*fed a majority of tinned food in their diet
+
 
*living strictly indoors, using litter
+
Extensive epidemiological studies over the last two decades have revealed that certain factors appear to contribute to feline hyperthyroidism:
*having a reported exposure to lawn herbicides, fertilisers and pesticides
+
*Age - the majority of cases occur in cats over 10 years of age
*having been regularly treated with flea sprays and powders
+
*Dramatic disparities in iodine concentrations in feline tinned foods<ref>Edinboro CH ''et al'' (2013) Iodine concentration in commercial cat foods from three regions of the USA, 2008-2009. ''J Feline Med Surg'' [http://www.ncbi.nlm.nih.gov/pubmed/23439761 Feb 25]</ref>
 +
*[[Goitrogenic compounds]] - primarily in commercial food<ref>Peterson M (2012) Hyperthyroidism in cats: what's causing this epidemic of thyroid disease and can we prevent it? ''J Feline Med Surg'' '''14(11)''':804-818</ref>, household cleaners, and topical insecticides<ref>August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA. pp:211-212</ref>
 +
 
 +
The combination of persist exposure to goitrogens repeatedly over time, often years, appears to be the primary etiological agent of this disease in cats. The [[pathogenesis of hyperthyroidism]], especially due to thyroid adenomas, is complex and not entirely clear, but factors such as dietary iodine content, autoimmune thyroiditis and lymphocytic thyroiditis have been disproved or no longer in flavor as theories to explain hyperfunctional thyroid tissue.
 +
 
 +
A complicating issue associated with this disease is hyperthyroid-masking of underlying [[chronic renal disease]] due to hyperthyroid induced increase in renal blood flow and glomerular filtration rate, masking a potential [[azotemia]]<ref>Williams TL ''et al'' (2010) Survival and the development of azotemia after treatment of hyperthyroid cats. ''J Vet Intern Med'' '''24(4)''':863-869</ref>.
  
Goitrogenic compounds including phthalates, resorcinol, polyphenols (plasticised bisphenol A - BPA) and PCBs. Cats have a relatively slow capacity for glucuronidation, the metabolic pathway responsible for metabolising many goitrogenic compounds. Autoantibodies have been suggested as a risk factor for the development of hyperthyroidism, but have not been proven to exist in hyperthyroid cats. A genetic basis for hyperthyroidism also has been suggested. Decreased expression of a G-protein in adenomatous thyroid glands of some hyperthyroid cats has been shown to reduce the negative inhibition of the cAMP cascade in thyroid cells. This leads to autonomous growth of the thyroid and the hypersecretion of thyroxine. The results of one study indicated that over-expression of the c-ras oncogene in hyperthyroid cats was highly associated with areas of nodular follicular hyperplasia and adenomas of the thyroid glands. Currently, the precise aetiology of hyperthyroidism is unclear<ref>August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA</ref>. 
 
 
== Clinical signs ==
 
== Clinical signs ==
Hyperthyroidism affects older cats most commonly. It is seen occasionally in cats as young as 4 years of age. The clinical signs include weight loss, increased activity, polyphagia, vomiting or diarrhoea, increased vocalisation, increased drinking and increased urination. The increase in thyroid hormone causes the cat's heart to beat faster (often > 240 beats per minute). Heart murmurs may be present. Heart damage is a common result of this disease, although it is usually reversible with treatment. The coat may look scruffy. Enlarged thyroid glands may be found. Effects on the kidneys from the circulatory changes can make existing kidney disease worse or cause the appearance of kidney disease in some cats.
+
Hyperthyroidism affects older cats most commonly. It is seen occasionally in cats as young as 4 years of age. The clinical signs include weight loss, increased activity, polyphagia, vomiting or diarrhoea, increased vocalisation, increased drinking and increased urination.  
 +
 
 +
Much is written concerning hyperthyroidism as a cause of [[hypertension]], [[chronic renal disease]] and [[congestive heart failure|cardiac dysfunction]]. It is difficult to critically review the published literature, as many cats with hyperthyroidism have concurrent renal and cardiac disease that may or may not have a causal relationship with their endocrinopathy. It is well established that hyperthyroidism is associated with increased glomerular filtration rates (GFR) in cats, and that GFR declines after treatment of hyperthyroidism. Although the exact mechanism is not known, it is likely that increased cardiac output and decreased peripheral vascular resistance associated with hyperthyroidism cause increased GFR by enhancing renal plasma flow<ref>Bradley, SE ''et al'' (1974) The thyroid and the kidney. ''Kidney Int'' '''6''':346</ref>.
 +
 
 +
Hyperthyroidism ''per se'' is rarely a cause of symptomatic hypertension; said another way, most cats with hypertension and hyperthyroidism have persistent hypertension after their thyroid disease is controlled. Cardiac disease is quite different, in that there is no doubt that hyperthyroidism induces a high cardiac output state, which in time can result in direct and indirect damage to the myocardium and eventually signs of congestive heart failure.
 +
 
 +
Cats which develop CHF as a result of thyrotoxicosis have a characteristic echocardiographic picture, with features of both hypertrophic and dilatative cardiomyopathy. In other words, they have biatrial dilatation, left ventricular hypertrophy, obvious ventricular chamber dilatation, and variable contractility (sometimes with a reduced fractional shortening)<ref>[[Prof Richard Malik|Malik R]] (1995) Feline hyperthyroidism -- an opinionated perspective ''The Veterinarian''</ref>.
  
 
== Diagnosis ==
 
== Diagnosis ==
Feline hyperthyroidism was first diagnosed in 1979, when clinical reports of its existence began to emerge. This disease has been diagnosed with increased frequency since that time, and it is now considered the most common endocrine disorder of cats. The increased frequency of diagnosis of feline hyperthyroidism can be attributed to increased clinical awareness of the disease, improved diagnostic testing, an increasing feline population, increased lifespan of pet cats, and the fact that more owners seek veterinary aid for their pets.  
+
The diagnosis of feline hyperthyroidism by veterinarians usually requires the combination of a detailed medical history, thorough physical examination, and confirmation of disease via laboratory testing. The medical history should note any changes in activity, behaviour, or appearance that are suggestive of hyperthyroidism. Hyperthyroidism is commonly diagnosed by showing an elevated level of T4 in the blood. However, cats with hyperthyroidism can have normal serum thyroid hormone concentrations, normal hematocrits, and normal serum concentrations of creatinine despite the presence of disease that affects these parameters<ref>Graves TK (2011) When normal is abnormal: keys to laboratory diagnosis of hidden endocrine disease. ''Top Companion Anim Med'' '''26(2)''':45-51</ref>.
  
 
Testing for hyperthyroidism is done by;
 
Testing for hyperthyroidism is done by;
* measuring the resting [[total thyroxine (T4) and triiodothyronine (T3) levels]] in the blood. The total T4 is the first test used to assess thyroid function. In most cats with hyperthyroidism, these levels will be above normal. Total T4 values may fall within normal reference ranges in early hyperthyroidism, or where there is concurrent non-thyroidal illness present ('euthyroid sick syndrome). In house testing of T4 has been shown to be inaccurate for measurement of real T4 in cats. Free T4 can be helpful in diagnosing hyperthyroidism in a patient with high normal T4 along with clinical signs suggestive of hyperthyroidism. It must be noted that non-thyroidal illness can (in <1% of cats) cause artificial elevation of T4 resulting in misdiagnosis.
+
*[[Thyroid manual palpation]] is important in the detection of thyroid gland abnormalities. 
* [[thyrotropin-releasing hormone (TRH) stimulation test]] - has good accuracy but there is a 50% chance of transient malaise during testing.
+
*Total [[thyroxine (T4)]] and triiodothyronine (T3) levels - the total T4 is the first test used to assess thyroid function. In most cats with hyperthyroidism, these levels will be above normal. Total T4 values may fall within normal reference ranges in early hyperthyroidism, or where there is concurrent non-thyroidal illness present ('euthyroid sick syndrome). In house testing of T4 has been shown to be inaccurate for measurement of real T4 in cats. Free T4 can be helpful in diagnosing hyperthyroidism in a patient with high normal T4 along with clinical signs suggestive of hyperthyroidism. It must be noted that non-thyroidal illness can (in <1% of cats) cause artificial elevation of T4 resulting in misdiagnosis.
 +
* [[Thyrotropin-releasing hormone (TRH) stimulation test]] - has good accuracy but there is a 50% chance of transient malaise during testing.
 
* [[T3-suppression test]] - a relatively easy test to perform but has a grey zone in that it is unclear which results signify hyperthyroidism or not.
 
* [[T3-suppression test]] - a relatively easy test to perform but has a grey zone in that it is unclear which results signify hyperthyroidism or not.
* Technetium scanning - requires a facility and personnel able to perform and interpret radio-uptake. This has led to the revelation that 20-25% of hyperthyroid cats have ectopic functional thyroid tissue located elsewhere (often intrathoracic) and thus warrants investigation and often precludes use of surgical intervention (Harvey ''et al'', 2009).
+
* [[TSH stimulation test]] - attempts to measure feline TSH in hyperthyroid cats using commercially available canine TSH assays have not shown the necessary sensitivity to clearly distinguish between low and normal concentrations.
* Thyroid Stimulating Hormone (TSH) response test - attempts to measure feline TSH in hyperthyroid cats using commercially available canine TSH assays have not shown the necessary sensitivity to clearly distinguish between low and normal concentrations.
+
* [[Radioisotope imaging|Radioisotope scanning/Technetium scanning]] - requires a facility and personnel able to perform and interpret radio-uptake. This has led to the revelation that 20-25% of hyperthyroid cats have ectopic functional thyroid tissue located elsewhere (often intrathoracic) and thus warrants investigation and often precludes use of surgical intervention.
 
 
The diagnosis of feline hyperthyroidism by veterinarians usually requires the combination of a detailed medical history, thorough physical examination, and confirmation of disease via laboratory testing. The medical history should note any changes in activity, behaviour, or appearance that are suggestive of hyperthyroidism.
 
Thyroid palpation is important in the detection of thyroid gland abnormalities. Palpation is best performed by raising the cat’s chin to a 45º angle and turning the head 45º to the right, placing the left index finger in the groove between the trachea and muscles to the left of the larynx. The index finger should be moved downward to the thoracic inlet. The direction of the head should be reversed and palpation repeated to examine the right cervical area. Palpation can be highly sensitive in detecting hyperthyroid cats, but many euthyroid cats also may possess palpable goitres. However, a large number of elderly, euthyroid cats with palpable goitres ultimately develop hyperthyroidism<ref>Harvey AM ''et al'' (2009) Scintigraphic findings in 120 hyperthyroid cats. ''JFMS'' '''11''':96-106</ref>.  
 
  
 
Hyperthyroid heart disease and cardiac disturbances also are quite common in hyperthyroid cats. These changes include tachycardia (rapid heart rate), murmurs, premature beats, or gallop rhythms. These findings generally are attributed to the high-output cardiac state caused by the effect of excess thyroid hormone on cardiac muscle as well as its effects on the sympathetic nervous system
 
Hyperthyroid heart disease and cardiac disturbances also are quite common in hyperthyroid cats. These changes include tachycardia (rapid heart rate), murmurs, premature beats, or gallop rhythms. These findings generally are attributed to the high-output cardiac state caused by the effect of excess thyroid hormone on cardiac muscle as well as its effects on the sympathetic nervous system
  
 
== Treatment ==
 
== Treatment ==
Because an irreversible decline in renal function occurs with many cases of definitive treatment of hyperthyroidism, it may be prudent to assess renal function before and after the hyperthyroid state has been corrected. There is anecdotal evidence suggesting that cats with normal blood urea and creatinine and USG > 1.035 having a reduced risk for the development of renal insufficiency after treatment for hyperthyroidism. Pretreatment GFR is also reported to be a predictor of post-treatment renal failure, with one study reporting that a pre-treatment GFR of less than 2.25 ml/kg/min was 100% sensitive and 78% specific for post-treatment renal failure. However, none of these predictors withstand thorough investigation and it is best to manage each case on an ad hoc basis<ref>[http://www.vet.uga.edu/vpp/clerk/stortz/index.htm Stortz, JS (2009) et al]</ref>.
+
Because an irreversible decline in renal function occurs with many cases of definitive treatment of hyperthyroidism, it may be prudent to assess renal function before and after the hyperthyroid state has been corrected.  
 +
 
 +
There is also evidence suggesting that cats with normal blood urea and creatinine and USG > 1.035 having a greater risk of developing [[chronic renal disease]] after treatment for hyperthyroidism<ref>Williams TL ''et al'' (2010) Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. ''J Vet Intern Med'' '''24(5)''':1086-1092</ref>.  
 +
 
 +
Pretreatment GFR is also reported to be a predictor of post-treatment renal failure, with one study reporting that a pre-treatment GFR of less than 2.25 ml/kg/min was 100% sensitive and 78% specific for post-treatment renal failure. However, none of these predictors withstand thorough investigation and it is best to manage each case on an ad hoc basis<ref>[http://www.vet.uga.edu/vpp/clerk/stortz/index.htm Stortz, JS (2009) et al]</ref>.
 +
 
 +
There are numerous medical methods for managing hyperthyroidism.
 +
*[[Carbimazole]] - popular oral treatment in Australia, Canada and Europe. Active metabolite of [[methimazole]], with fewer reported side-effects.
 +
*[[Methimazole]] - popular oral treatment in the United States for oral treatment of hyperthyroidism
 +
*[[Ipodate]] - an oral medication option in cats intolerant of methimazole or carbimazole. This drug may be beneficial for acute management of thyrotoxicosis in some cats, but is not suitable for long-term management<ref>Gallagher, AE & Panciera, DL (2011) Efficacy of iopanoic acid for treatment of spontaenous hyperthyroidism in cats. ''JFMS'' '''13''':441-447</ref>.
 +
*[[Radiotherapy]] - radioactive iodine(<sup>131</sup>I) therapy remains the gold standard for patients in which oral administration is unsuccesful<ref>van Hoek IM ''et al'' (2009) Effect of recombinant human thyroid stimulating hormone on serum thyroxin and thyroid scintigraphy in euthyroid cats. ''JFMS'' '''11''':309-314</ref>.
 +
*[[Thyroidectomy]] - an effective surgical procedure in most cats, but technically demanding.
 +
*[[Percutaneous intrathyroidal ethanol injection]] - primarily for unilateral hyperthyroidism, high skill required, not recommended
 +
*[[Percutaneous intrathyroidal heat ablation]] - effective short-term treatment, an experimental procedure only
 +
*Herbal supplements - Thyroidinum has shown to resolve clinical signs in three of four cases in cats<ref>Chapman SF (2012) Homeopathic and integrative treatment for feline hyperthyroidism--four cases (2006-2010). ''Homeopathy'' '''100(4)''':270-274</ref>
 +
 
 +
Because treatment of hyperthyroidism has the potential to reveal pre-existing chronic renal disease and create iatrogenic [[hypothyroidism]], methimazole should be administered in animals with possible renal compromise.
 +
 
 +
If worsening of renal failure occurs, hyperthyroidism should not be treated or should be treated with methimazole to bring the T4 into the range of 5 - 6 ug/dL or to the lowest achievable level that stops weight loss and does not cause worsening of the [[azotemia]]<ref>Norsworthy, GD ''et al'' (2006) The feline patient. 3rd edition. Blackwell Publishing, Iowa, p:149</ref>.
  
There are currently three commonly used treatments for this problem.
+
Once a euthyroid state has been achieved with chemotherapy, surgery or radiation therapy, regular quarterly blood testing of T4 should be performed for the life of the cat<ref>Higgs P & Hibbert A (2012) Managing hyperthyroidism in cats. ''Vet Rec'' '''171(9)''':225-226</ref>.
*Medication
 
:'''[[Methimazole]]''' - Transdermal methimazole has been shown to be absorbed but that it may take four weeks of use to get to therapeutic serum levels. Once daily administration by any route is not as effective as twice daily use and cannot be recommended.
 
:'''[[Carbimazole]]''' (Neomercazole®) - the active metabolite of methimazole which is reported to have fewer side effects than methimazole. First choice of treatment as it quickly reduces the level of thyroid hormone in the blood, and clinical symptoms usually disappear within 4 weeks. Regular blood testing every 6-8 weeks is required to monitor thyroid levels.
 
:'''[[Ipodate]]''' (Orograffin) or iopanoic acid (Telepaque) may be options in cats requiring oral therapy but who cannot tolerate methimazole or carbimazole.
 
:Other treatments that have been recently evaluated are ethanol injection of the thyroid and percutaneous heat ablation of the thyroid. The former cannot be recommended because of serious adverse effects and the latter is not a permanent solution.
 
*[[Radiotherapy]]
 
Radioiodine (<sup>131</sup>I) therapy remains the gold standard, although iatrogenic hypothyroidism  can occur after <sup>131</sup>I treatment in 6-30% of cases (van Hoek ''et al'', 2009). It is effective in about 90% of cats. For patients in which the daily administration of pills is undesirable this is the best method of treatment. The major disadvantage to radioactive iodine therapy is the costs, and it also requires the isolation of your cat at the vet hospital for 7 to 14 days due to safety concerns<ref>van Hoek IM ''et al'' (2009) Effect of reconbinant human thyroid stimulating hormone on serum thyroxin and thyroid scintigraphy in euthyroid cats. ''JFMS'' '''11''':309-314</ref>.
 
*Surgery
 
An effective procedure in most cats and usually suggested by your vet when medication no longer controls the symptoms.
 
  
In older cats, this disease is common enough that routine screening is considered to be necessary by many veterinarians. Due to the potential for numerous secondary complications, such as heart disease and digestive problems, early diagnosis is a good idea.
 
==Further reading==
 
[[media:Hyperthyroidism.pdf|Caring for a cat with hyperthyroidism (.pdf)]]
 
 
== References ==
 
== References ==
 
<References/>
 
<References/>

Latest revision as of 00:15, 3 April 2013

File:Hyper1.jpg
Cachexia in an Abyssinian associated with hyperthyroidism
File:Thyroid4.jpg
Feline hyperthyroidism, thyroid nodule
Enlarged thyroid gland in a Domestic shorthair cat
Thyroidectomy being performed on above cat
File:Hyper3.jpg
Lateral and ventrodorsal radiographs of a cat with hyperthyroidism and hyperthyroid heart disease. Notice the enlarged cardiac silhouette

Hyperthyroidism is the most common endocrine disorder affecting the thyroid gland of older cats, characterized classically by elevated total thyroxine (T4) above the reference range of 20-40 nmol/L.

Iodine content of the diet has been suggested as a likely etiologic factor in feline hyperthyroidism[1].

It is uncommon to see hyperthyroidism in cats under 10 years of age, and is commonly caused by a benign thyroid adenoma or adenomatous hyperplasia present in one or both lobes of the thyroid gland. Fewer than 2% of cases occur as a result of a functional thyroid carcinoma[2].

Extensive epidemiological studies over the last two decades have revealed that certain factors appear to contribute to feline hyperthyroidism:

  • Age - the majority of cases occur in cats over 10 years of age
  • Dramatic disparities in iodine concentrations in feline tinned foods[3]
  • Goitrogenic compounds - primarily in commercial food[4], household cleaners, and topical insecticides[5]

The combination of persist exposure to goitrogens repeatedly over time, often years, appears to be the primary etiological agent of this disease in cats. The pathogenesis of hyperthyroidism, especially due to thyroid adenomas, is complex and not entirely clear, but factors such as dietary iodine content, autoimmune thyroiditis and lymphocytic thyroiditis have been disproved or no longer in flavor as theories to explain hyperfunctional thyroid tissue.

A complicating issue associated with this disease is hyperthyroid-masking of underlying chronic renal disease due to hyperthyroid induced increase in renal blood flow and glomerular filtration rate, masking a potential azotemia[6].

Clinical signs

Hyperthyroidism affects older cats most commonly. It is seen occasionally in cats as young as 4 years of age. The clinical signs include weight loss, increased activity, polyphagia, vomiting or diarrhoea, increased vocalisation, increased drinking and increased urination.

Much is written concerning hyperthyroidism as a cause of hypertension, chronic renal disease and cardiac dysfunction. It is difficult to critically review the published literature, as many cats with hyperthyroidism have concurrent renal and cardiac disease that may or may not have a causal relationship with their endocrinopathy. It is well established that hyperthyroidism is associated with increased glomerular filtration rates (GFR) in cats, and that GFR declines after treatment of hyperthyroidism. Although the exact mechanism is not known, it is likely that increased cardiac output and decreased peripheral vascular resistance associated with hyperthyroidism cause increased GFR by enhancing renal plasma flow[7].

Hyperthyroidism per se is rarely a cause of symptomatic hypertension; said another way, most cats with hypertension and hyperthyroidism have persistent hypertension after their thyroid disease is controlled. Cardiac disease is quite different, in that there is no doubt that hyperthyroidism induces a high cardiac output state, which in time can result in direct and indirect damage to the myocardium and eventually signs of congestive heart failure.

Cats which develop CHF as a result of thyrotoxicosis have a characteristic echocardiographic picture, with features of both hypertrophic and dilatative cardiomyopathy. In other words, they have biatrial dilatation, left ventricular hypertrophy, obvious ventricular chamber dilatation, and variable contractility (sometimes with a reduced fractional shortening)[8].

Diagnosis

The diagnosis of feline hyperthyroidism by veterinarians usually requires the combination of a detailed medical history, thorough physical examination, and confirmation of disease via laboratory testing. The medical history should note any changes in activity, behaviour, or appearance that are suggestive of hyperthyroidism. Hyperthyroidism is commonly diagnosed by showing an elevated level of T4 in the blood. However, cats with hyperthyroidism can have normal serum thyroid hormone concentrations, normal hematocrits, and normal serum concentrations of creatinine despite the presence of disease that affects these parameters[9].

Testing for hyperthyroidism is done by;

  • Thyroid manual palpation is important in the detection of thyroid gland abnormalities.
  • Total thyroxine (T4) and triiodothyronine (T3) levels - the total T4 is the first test used to assess thyroid function. In most cats with hyperthyroidism, these levels will be above normal. Total T4 values may fall within normal reference ranges in early hyperthyroidism, or where there is concurrent non-thyroidal illness present ('euthyroid sick syndrome). In house testing of T4 has been shown to be inaccurate for measurement of real T4 in cats. Free T4 can be helpful in diagnosing hyperthyroidism in a patient with high normal T4 along with clinical signs suggestive of hyperthyroidism. It must be noted that non-thyroidal illness can (in <1% of cats) cause artificial elevation of T4 resulting in misdiagnosis.
  • Thyrotropin-releasing hormone (TRH) stimulation test - has good accuracy but there is a 50% chance of transient malaise during testing.
  • T3-suppression test - a relatively easy test to perform but has a grey zone in that it is unclear which results signify hyperthyroidism or not.
  • TSH stimulation test - attempts to measure feline TSH in hyperthyroid cats using commercially available canine TSH assays have not shown the necessary sensitivity to clearly distinguish between low and normal concentrations.
  • Radioisotope scanning/Technetium scanning - requires a facility and personnel able to perform and interpret radio-uptake. This has led to the revelation that 20-25% of hyperthyroid cats have ectopic functional thyroid tissue located elsewhere (often intrathoracic) and thus warrants investigation and often precludes use of surgical intervention.

Hyperthyroid heart disease and cardiac disturbances also are quite common in hyperthyroid cats. These changes include tachycardia (rapid heart rate), murmurs, premature beats, or gallop rhythms. These findings generally are attributed to the high-output cardiac state caused by the effect of excess thyroid hormone on cardiac muscle as well as its effects on the sympathetic nervous system

Treatment

Because an irreversible decline in renal function occurs with many cases of definitive treatment of hyperthyroidism, it may be prudent to assess renal function before and after the hyperthyroid state has been corrected.

There is also evidence suggesting that cats with normal blood urea and creatinine and USG > 1.035 having a greater risk of developing chronic renal disease after treatment for hyperthyroidism[10].

Pretreatment GFR is also reported to be a predictor of post-treatment renal failure, with one study reporting that a pre-treatment GFR of less than 2.25 ml/kg/min was 100% sensitive and 78% specific for post-treatment renal failure. However, none of these predictors withstand thorough investigation and it is best to manage each case on an ad hoc basis[11].

There are numerous medical methods for managing hyperthyroidism.

  • Carbimazole - popular oral treatment in Australia, Canada and Europe. Active metabolite of methimazole, with fewer reported side-effects.
  • Methimazole - popular oral treatment in the United States for oral treatment of hyperthyroidism
  • Ipodate - an oral medication option in cats intolerant of methimazole or carbimazole. This drug may be beneficial for acute management of thyrotoxicosis in some cats, but is not suitable for long-term management[12].
  • Radiotherapy - radioactive iodine(131I) therapy remains the gold standard for patients in which oral administration is unsuccesful[13].
  • Thyroidectomy - an effective surgical procedure in most cats, but technically demanding.
  • Percutaneous intrathyroidal ethanol injection - primarily for unilateral hyperthyroidism, high skill required, not recommended
  • Percutaneous intrathyroidal heat ablation - effective short-term treatment, an experimental procedure only
  • Herbal supplements - Thyroidinum has shown to resolve clinical signs in three of four cases in cats[14]

Because treatment of hyperthyroidism has the potential to reveal pre-existing chronic renal disease and create iatrogenic hypothyroidism, methimazole should be administered in animals with possible renal compromise.

If worsening of renal failure occurs, hyperthyroidism should not be treated or should be treated with methimazole to bring the T4 into the range of 5 - 6 ug/dL or to the lowest achievable level that stops weight loss and does not cause worsening of the azotemia[15].

Once a euthyroid state has been achieved with chemotherapy, surgery or radiation therapy, regular quarterly blood testing of T4 should be performed for the life of the cat[16].

References

  1. Mumma, RO et al (1986) Toxic and protective constituents in pet foods. Am J Vet Res 47:1633-1637
  2. Martin, KM et al (2000) Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. J Am Vet Med Assoc 217:853-856
  3. Edinboro CH et al (2013) Iodine concentration in commercial cat foods from three regions of the USA, 2008-2009. J Feline Med Surg Feb 25
  4. Peterson M (2012) Hyperthyroidism in cats: what's causing this epidemic of thyroid disease and can we prevent it? J Feline Med Surg 14(11):804-818
  5. August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA. pp:211-212
  6. Williams TL et al (2010) Survival and the development of azotemia after treatment of hyperthyroid cats. J Vet Intern Med 24(4):863-869
  7. Bradley, SE et al (1974) The thyroid and the kidney. Kidney Int 6:346
  8. Malik R (1995) Feline hyperthyroidism -- an opinionated perspective The Veterinarian
  9. Graves TK (2011) When normal is abnormal: keys to laboratory diagnosis of hidden endocrine disease. Top Companion Anim Med 26(2):45-51
  10. Williams TL et al (2010) Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med 24(5):1086-1092
  11. Stortz, JS (2009) et al
  12. Gallagher, AE & Panciera, DL (2011) Efficacy of iopanoic acid for treatment of spontaenous hyperthyroidism in cats. JFMS 13:441-447
  13. van Hoek IM et al (2009) Effect of recombinant human thyroid stimulating hormone on serum thyroxin and thyroid scintigraphy in euthyroid cats. JFMS 11:309-314
  14. Chapman SF (2012) Homeopathic and integrative treatment for feline hyperthyroidism--four cases (2006-2010). Homeopathy 100(4):270-274
  15. Norsworthy, GD et al (2006) The feline patient. 3rd edition. Blackwell Publishing, Iowa, p:149
  16. Higgs P & Hibbert A (2012) Managing hyperthyroidism in cats. Vet Rec 171(9):225-226