Hyperthyroidism is the most common hormonal disease of cats. The overproductionof thyroid hormone can be the result of hyperplasia (increased activityof the gland for unknown reasons) or cancer. A small percentage of thecancers are malignant. This disease was not commonly recognized prior tothe late 1970s. The reason for the increase in prevalence is not known.

Hyperthyroidism affects  older cats most commonly. It is seen occasionallyin cats as young as 4 years of age. The clinical signs include weight loss,increased activity, increased appetite, vomiting or diarrhea, increasedvocalization, increased drinking and increased urination. In some cats,the disorder produces atypical signs such as depression, inappetance orweakness.

The increase in thyroid hormone causes the cat's heart to beat faster,often > 240 beats per minute. Heart murmurs may be present. Heart failure will occur in up to 10% of cats and heart damage occurs in most, althoughit is usually reversible with treatment of the hyperthyroidism. The haircoatmay look scruffy. Enlarged thyroid glands may be found. Effects on thekidneys from the circulatory changes can make existing kidney disease worseor cause the appearance of kidney disease in some cats.

Diabetes, kidney disease, liver disease and other conditions that alsoaffect older cats need to be ruled out prior to settling on a diagnosisof hyperthyroidism, even if tests indicate it is present.

Testing for hyperthyroidism is done by measuring the T4 (one thyroidhormone) levels in the blood stream. In most cats with hyperthyroidism,these levels will be above normal. In some cats, they will be in the "normal"range, despite the presence of the disease. This is particularly true ofvery old cats. In these cats, repeating the test in a week or so is oftendiagnostic. If not, more specialized testing must be done, such as T3 (anotherthyroid hormone) suppression tests or thyroid releasing hormone (TRH) testing.

There are currently three commonly used treatments for this problem. Surgery, radioactive iodine therapy and medical treatment using methimazole (Tapazole (rx)).

Medical treatment may be effective long term if the cat is compliantabout taking pills and no side effects occur. It is not uncommon to see side effects from methimazole, which can include anemia and decreased plateletnumbers. Most side effects occur within the first month. Medical treatmentis less costly in the short run but over the lifetime of the cat, it maybe more expensive than alternative methods of treating for this problem.

Surgery is an effective procedure in most cats. There is a higher thannormal risk of complications with surgery on the thryoid gland, due tothe parathyroid glands in the region. These glands control calcium regulationin the body and they are easily damaged during surgery. Death can resultif calcium levels drop sufficiently. Therefore, calcium levels should becarefully monitored for a week if both of the thyroid glands are affected.

Radioactive iodine therapy is probably the best combination of safetyand efficiency for treatment of hyperthyroidism. It is effective in about90% of cats, no surgery or anesthesia is required and the parathyroid glandsare not affected. For patients in which the daily administration of pillsis undesirable this is the best method of treatment. The major disadvantageto this therapy is the required isolation of the cat at a treatment facilityfor 7 to 14 days following the administration of the radioactive iodine,due to safety concerns.

In older cats, this disease is common enough that routine screeningis considered to be necessary by many veterinarians. Due to the potentialfor numerous secondary complications, such as heart disease and digestiveproblems, early diagnosis is a good idea. Consider asking your vet abouttests for hyperthyroidism if your cat is over 10 years of age.