Feline hyperthyroidism is a disorder resulting from over activity of the thyroid gland. It is a relatively newly recognised disease in veterinary medicine. In the late 1970's a few cases of hyperthyroidism in cats were discovered in the USA. Over the next ten years, the number of cats affected by hyperthyroidism in the USA increased from a few cases per year to a few cases per month and then increased to a daily diagnosis at some hospitals. Europe and some other areas followed a similar pattern. In South Africa we may now be following the same pattern as we recognise more and more patients with hyperthyroidism. Just as in the USA and Europe, this is not merely as result of increased recognition of the problem, but an actual increase in the number of patients affected by the disorder. The reasons for this increase in hyperthyroidism are not known. Possible causes include environmental factors, nutritional factors or internal immune mediated factors. Interestingly, dogs are not affected, only cats.
Hyperthyroidism affects primarily the senior cat, usually older than 10 years. The production of excessive amounts of thyroid hormone affects the entire body function, and may result in one or more of a number of clinical signs. The most common symptoms may include weight loss despite a very good or increased appetite, increased activity levels, vomiting, increased drinking, diarrhoea, weakness, panting, and an unkempt appearance and lack of proper grooming. Some cats may rather have a poor appetite and decreased activity levels (so-called apathetic hyperthyroidism). During physical examination your veterinarian may recognise the weight loss, unkempt appearance, fast heart rate, panting or breathing problems, heart murmur, and lack of co-operation or even aggression. Typically, affected patients cannot sit still for longer than a few seconds and will absolutely resist confinement or handling. The most important clinical finding is palpable enlargement of the thyroid glands in the neck.
Blood tests can be performed to confirm the diagnosis of hyperthyroidism. While liver and kidney function tests may be slightly abnormal, a specific thyroid test can be performed and will usually indicate excessive thyroid hormone levels. Occasionally the test may need to be repeated after a few weeks to confirm the diagnosis.
Luckily, while the disease may be present in the patient for a very long period and can ultimately be very debilitating or even result in fatal heart failure, it is also very amenable to treatment. Various treatments are available. Specific treatment using an antithyroid drug is very successful, but it must be administered orally, two or three times daily, for continued effect. In our experience, surgical removal of the affected thyroid glands is a very successful and rewarding treatment, and afterwards most patients will usually need no further oral medication. There are some initial dangers involved in the surgery, and the surgery should therefore only be performed after stabilisation of the condition for a few weeks using the oral medication. Occasionally some patients may need additional treatment for heart failure. Administration of radioactive iodine to destroy the affected thyroid glands is a commonly utilised treatment form in the USA, but is not freely available here.
Because the condition is still uncommon in our country, it may easily escape detection during routine examination. Be on the lookout for the above signs and alert your veterinarian if you do recognise the symptoms - you may save your cat's life!