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Hyperthyroidism is an endocrine disease resulting from excess secretion of thyroid hormone from the thyroid gland(s). Older cats are most typically affected. Both males and females and any breed are at risk. Most commonly primary disease or functional adenomatous goiter of the thyroid gland(s) is the cause. Usually both glands are affected. Less commonly ectopic thyroid tissue can be the cause.

The clinical signs most suggestive of hyperthyroidism are weight loss, increased appetite, hyperactivity, excess vocalization, an unkempt appearance, vomiting/diarrhea and/or an increase in drinking and urination. A minority of cats will be atypical; lethargic and anorexic. Some cats show mainly a behavior change and can become quite aggressive.

A strong suspicion of hyperthyroidism usually arises when a nodule low on the neck is felt on physical exam; although, all affected cats do not have palpable nodules and some cats with nodules are not affected. A routine complete blood cell count and chemistry screen can show changes consistent with hyperthyroidism, but cannot definitively diagnose the disease. Generally, a baseline serum T4 is considered the test of choice for definitive diagnosis; although, additional tests are recommended in some cases where only mild hyperthyroidism or other concurrent systemic disease may exist.

Treatment usually consists of oral medications initially (i.e., Methimazole/Tapazole/Felimazole) +/- more permanent therapy later on (radioactive iodine therapy or possibly surgery). Methimazole is a relatively safe medication, but mild to severe side effects can be seen. Anorexia, vomiting, facial excoriation, liver damage and decreases in blood cells are all possible side effects. Your veterinarian should be contacted if any side effects are noted. Routine monitoring is required, including lab tests.

The prognosis is generally good for hyperthyroid cats, without underlying/concurrent disease. With concurrent disease, the prognosis is largely based on the other disease process going on. Hyperthyroidism can mask signs of renal/kidney disease. If signs become apparent when treatment is initiated, a balance must be reached where the hyperthyroidism is controlled enough to decrease the clinical signs, but where the kidney disease does not become full-fledged failure. This can be a bit difficult to manage and the prognosis becomes poorer.

Overall, hyperthyroidism is not the cause of many emergency situations, but it can factor into the treatment and prognosis. For example, extreme weight loss and/or high blood pressure associated with poorly controlled hyperthyroidism could make a patient less ideal for emergency surgery. If a hyperthyroid cat is presented for an emergency, it is important to notify the doctor of his/her history.