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 Feline Hyperthyroidism


The thyroid glands are located in the neck and play a vital role in regulating the body's metabolic rate. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in metabolic rate and is a fairly common disease of older cats.


Many organs are affected, including the heart. The heart is stimulated to pump faster and more forcefully, eventually causing the heart to enlarge and often times leading to high blood pressure; about 25% of hyperthyroid cats have high blood pressure. 


A specific cause of hyperthyroidism is not known, though dietary factors may play a role. No individual breed is know to be especially at increased risk for hyperthyroidism, although the Siamese appears to have a somewhat increased incidence of it.


Some common clinical signs are weight loss, with an increase in appetite. In fact, some cats have such a ravenous appetite, they will literally eat anything in sight! These cats will often drink a lot of water and frequently urinate. There may be periodic vomiting or diarrhea, and their hair coat may be unkempt. As the disease progresses, some cats may actually begin to lose their appetite.


Diagnosis of the disease is fairly straightforward and is often diagnosed quite quickly with a simple blood test.


There are three common treatments for hyperthyroid cats:

  • Radioactive Iodine: It destroys abnormal thyroid tissue without endangering other organs. The treatment requires one or two weeks of hospitalization. This treatment option can often be expensive.

  • Surgery: Surgical removal of the affected thyroid lobe. If the cat is healthy, the risk for this procedure is minimal.

  • Medication: Administration of methimazole can control the effects of the overactive thyroid gland. Though the number is relatively small (20%), some cats can have reactions to the drug, like vomiting and lethargy. Methimazole does not destroy the tissue, it suppresses the production of excess thyroid hormone, so most cats receive the drug for the remainder of their lives.

The prognosis is often good for cats diagnosed with hyperthyroidism, and if it is properly treated or controlled, most cats live a long healthy life.