Hyperthyroidism is the overproduction of thyroid hormone by the thyroid glands. Hyperthyroidism occurs most commonly in older cats. The average age of cats with hyperthyroidism is 13 years of age with only about 5% younger than 10 years of age. There are 2 thyroid glands located in the neck. One or both of the glands can enlarge and overproduce thyroid hormone. Involvement of both glands is more common than involvement of one gland. Thyroid hormone affects the function of most organs in the body, so the clinical signs are quite variable.

Clinical Signs:
  • weight loss
  • increased appetite
  • increased activity and restlessness
  • aggressive or "cranky" behavior
  • poor hair coat
  • a fast heart rate
  • periodic vomiting
  • occasionally difficulty breathing
  • occasionally weakness
  • occasionally depression
A diagnosis is made when the level of thyroid hormone is increased in the blood. Most hyperthyroid cats have very high levels of hormone but some cats will have clinical signs with normal or only slightly increased levels.

The enlarged thyroid gland(s) can often be felt in the neck. If the diagnosis is not obvious by blood tests, other tests used to confirm diagnosis include measurement of free T4, the T3 suppression test, thyrotropin-releasing hormone stimulation test and thyroid radionuclide uptake and imaging.
A blood panel and urinalysis are also performed to screen for abnormalities in other organs such as liver and kidney that may be present due to the advanced age of the animal.

High levels of thyroid hormone may cause heart disease. The heart may appear enlarged on x-ray or ultrasound and show abnormal electrical activity on an ECG (electrocardiogram).
Cats with serious heart disease and hyperthyroidism need to be treated for both diseases.
There are three types of treatment for hyperthyroidism:
  • life long oral anti-thyroid medications
  • surgical removal of affected thyroid glands
  • treatment with radioactive iodine

All three treatments will reduce thyroid hormone levels and the signs of hyperthyroidism. If other diseases are present, one treatment may be better than the other.

The anti-thyroid pill (Methimazole) is given daily and must be continued lifelong. It takes several weeks for methimazole to reduce blood thyroid hormone levels to normal. If discontinued, thyroid hormone levels will return to high levels over a few weeks. Methimazole may be used to reduce thyroid hormone levels to normal before surgically removing the thyroid gland(s).

Enlarged thyroid glands can be surgically removed. Methimazole is given for 1 to 2 months before surgery so that thyroid hormone levels are normal at the time of surgery. If both glands are enlarged, they can both be removed and most cats will still produce enough thyroid hormone by a few thyroid cells scattered throughout the body to prevent hypothyroidism (abnormally low thyroid hormone levels).
A few cats may become hypothyroid and will need thyroid medication. Occasional complications include damage to the parathyroid glands, which are closely attached to the thyroid gland, damage to nerves close to the thyroid gland or damage to the voice box. Other complications include failure to resolve the hyperthyroidism. These cats have thyroid cells in abnormal locations, (ectopic thyroid). If surgery is elected, a nuclear medicine scan can be performed before surgery to determine if there is ectopic thyroid tissue. If ectopic thyroid tissue exist, then a different treatment, either methimazole or radioactive thyroid treatment should be selected.
Cats that have had surgery may have recurrence of hyperthyroidism. Blood thyroid hormone levels should be measured once or twice a year.

Treatment with radioactive iodine is given intravenously and will accumulate in the abnormal thyroid tissue killing the abnormal thyroid cells but sparing the normal thyroid cells.
Radioactive iodine will also accumulate in ectopic thyroid tissue. Radioactive iodine treatment is very effective and rarely causes hypothyroidism. The cats do not have to be placed under anesthesia for the procedure. The disadvantages include the need to travel to a facility that offers this treatment and that the cat to remain hospitalized until the level of radioactivity decreases to a safe level (usually 1 to 3 weeks). Older cats with hyperthyroidism often also have kidney disease. Treatment of these cats is a delicate balancing act.
Treatment of hyperthyroidism is usually successful and properly treated cats can lead normal healthy lives.