The cause of hyperthyroidism is not known but the majority are due to a benign tumour in the gland. Certain factors may contribute:

1. fed a majority of tinned food in their diet
2. living strictly indoors, using litter
3. having a reported exposure to lawn herbicides, fertilisers and pesticides
4. having been regularly treated with flea sprays and powders

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. 

Symptoms

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, increased appetite, 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.

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. Testing for hyperthyroidism is done by;

1. measuring thyroid hormone 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.
2. Thyrotropin Releasing Hormone (TSH) has good accuracy but there is a 50% chance of transient malaise during testing.
3. Triiodothyronine (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.
4. Technetium scanning - requires a facility and personnel able to perform and interpret radio-uptake
5. 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.
6. 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 (see Table).
7. 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. 

Figure 3. Lateral and ventrodorsal radiographs of a cat with hyperthyroidism and hyperthyroid heart disease. Notice the enlarged cardiac silhouette

Hyperthyroid heart disease and cardiac disturbances also are quite common in hyperthyroid cats (see Fig. 3). 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

An MRI can also be performed to test whether the thyroid activity is normal (see images below).

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.

There are currently three commonly used treatments for this problem.

    1. Medication

1. 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.
2. Carbimazole (Neomercazole®): Carbimazole is 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.
3. Ipodate (Orograffin) or iopanoic acid (Telepaque) may be options in cats requiring oral therapy but who cannot tolerate methimazole or carbimazole.
4. 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.
5. Radioiodine therapy remains the gold standard. 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.

    2. 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.