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Veterinary Teaching Hospital

Hyperthyroidism in cats

Close up image of British shorthair cat.

What is hyperthyroidism?

Hyperthyroidism is the overproduction of thyroid hormone by the thyroid glands. Cats have two thyroid glands located in their neck. One or, more commonly, both glands can enlarge and overproduce thyroid hormone.

The average age of cats with hyperthyroidism is 13; only about 5% of hyperthyroid cats are younger than 10.

What are some of the signs of hyperthyroidism in cats?

Thyroid hormone affects the function of most organs in the body and the signs are variable. Signs include:

  • weight loss
  • increased appetite
  • increased activity and restlessness
  • aggressive or “cranky” behavior
  • a poor coat
  • a fast heart rate
  • increased water drinking
  • increased urination
  • periodic vomiting
  • increased amount of stool or diarrhea
  • occasional difficulty breathing
  • occasional weakness
  • occasional depression

How is hyperthyroidism diagnosed in cats?

Hyperthyroidism can be detected by the presence of increased thyroid hormone in a blood sample. An enlarged thyroid gland can also often be felt in the neck. If the diagnosis is not obvious by blood tests, a nuclear medicine scan of the thyroid glands can be performed at certain specialty veterinary practices, like the WSU Veterinary Teaching Hospital.

While most hyperthyroid cats have very high levels of thyroid hormone, some will have signs with normal or only slightly increased levels of thyroid hormone. Thyroid hormone levels can vary over time, so it may be necessary to check blood levels several times or perform a T3 suppression test.

Since high levels of thyroid hormone may cause heart disease, the heart may appear enlarged on X-ray or ultrasound and may show abnormal electrical activity on an electrocardiogram.

How are cats with hyperthyroidism treated?

There are three types of treatments for hyperthyroidism:

  • life-long oral anti-thyroid medications
  • surgical removal of affected thyroid glands
  • radioactive iodine

All three treatments will reduce thyroid hormone levels and the signs of hyperthyroidism.

The anti-thyroid pill is methimazole, also known as Tapazole. It is given one to three times daily and must be continued for the remainder of the pet’s life. It takes several weeks for methimazole to reduce blood thyroid hormone levels to normal. If discontinued, levels will increase. Methimazole may be used to reduce thyroid hormone levels to normal before surgically removing a thyroid gland. Cats with heart disease may be too sick and fragile to anesthetize for surgery, but methimazole can be given until the heart improves and the cat is stronger. Methimazole may produce side effects in cats, including depression, vomiting, and lack of appetite. These signs usually resolve without stopping the medication. A more serious side effect is the development of low blood cell counts, which is more likely to develop during the first three months of treatment.

Surgical removal of the thyroid gland(s) can usually be performed without complications. If both glands are enlarged and removed, 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 will become hypothyroid and may need to take thyroid pills.

Occasionally, complications may develop, including 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. Parathyroid gland damage causes low blood calcium that may lead to seizures. Low blood calcium is treated with calcium or vitamin D. Nerve damage causes abnormal size of the pupils of the eyes and droopy eyelids.

Some cats will remain hyperthyroid after surgical removal of the thyroid glands. These cats have thyroid cells in abnormal locations where surgical removal can be difficult. This extra thyroid tissue is called ectopic thyroid. If you and your veterinarian decide that surgery is the best treatment option for your cat, a nuclear medicine scan can be performed before surgery to see if your cat has ectopic thyroid tissue. If ectopic thyroid tissue is seen on the nuclear medicine scan, then a different treatment should be selected.

Cats that have had surgery may have a recurrence of hyperthyroidism. Blood thyroid hormone levels should be measured once or twice a year.

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 disadvantages of radioactive iodine treatment include the need to travel to a facility that offers this treatment and the need for the cat to remain hospitalized until the level of radioactivity decreases to a safe level as determined by the state radiation control office (usually 1 to 3 weeks).

Older cats with hyperthyroidism often also have kidney disease. Treatment of these cats is a delicate balancing act. Hyperthyroidism can actually improve kidney function by increasing blood flow to the kidneys, and some cats with kidney disease will show a worsening of kidney function after treatment for hyperthyroidism. Talk to your veterinarian about monitoring kidney function in hyperthyroid cats.

What should I do if my cat is showing symptoms of hyperthyroidism?

If your cat is showing symptoms of hyperthyroidism, you should consult with your veterinarian or call the WSU Veterinary Teaching Hospital at 509-335-0711. If your cat has already been diagnosed with the condition, veterinarians at WSU can discuss treatment options with you.


This information is not meant to be a substitute for veterinary care. Always follow the instructions provided by your veterinarian. Washington State University assumes no liability for injury to you or your pet incurred by following these descriptions or procedures.