Hyperthyroid Cats

Dr. Louis N. Gotthelf (member American Academy of Veterinary Dermatology)
Montgomery Pet Skin and Ear Clinic
3310 Atlanta Highway
Montgomery, AL 36109 

Feline hyperthyroidism is a disorder resulting from excessive thyroid hormone levels, most often from a thyroid gland tumor.

Though functional benign enlargement (adenoma) is most common (98%), thyroid carcinoma (cancer) is another cause (2%). The average age of cats with hyperthyroidism is 13 years of age; only about 5 % of hyperthyroid cats are younger than 10 years of age.

There are 2 thyroid glands located in the neck. The enlarged thyroid gland(s) can often be felt along the trachea when they are enlarged. 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 signs of hyperthyroidism are quite variable.

Signs of hyperthyroidism in a cat can include:

  • weight loss and poor body condition
  • increased appetite with weight loss
  • increased activity and restlessness
  • aggressive or "cranky" behavior
  • a poor hair coat
  • a fast heart rate
  • increased water drinking
  • increased urination
  • periodic vomiting
  • increased amount of stool or diarrhea
  • occasionally difficulty breathing
  • occasionally weakness
  • occasionally depression

A diagnosis of hyperthyroidism is made when the level of thyroid hormone, called thyroxine or T4, is increased in the blood. We can measure the total T4 with a blood test in our office in about 10 minutes.

Most hyperthyroid cats have very high levels of hormone but some cats will have signs of hyperthyroidism with normal or only slightly increased levels of thyroid hormone.


A large thyroid tumor at surgery

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What can be done to treat Hyperthyroid Cats?

There are 4 basic methods of treating cats with high thyroid levels.

1. Hill's y/d food, a severely restricted iodine cat food. The idea is that if the thyroid gland does not have Iodine, it will not be able to produce much thyroid hormone.


2. Methimazole is a drug that inhibits the manufacture of the thyroid hormone thyroxin in the thyroid glands. In a tumorous gland, the excessive thyroid hormone secretion can be reduced. This treatment can be by tablet or by a transdermal gel applied to the outside surface of the ears.

3. Radioactive I-131 therapy. This is a treatment that is done at a university or a special clinic set up so that they can handle radioactive chemicals. The idea is that the radioactive Iodine is taken up by the thyroid tumor and the tumor cells die. The cat is hospitalized for about a week in a special lead lined room and then sent home when the radioactivity levels are safe. This can be done at Auburn University.

3. Surgery. Removal of one or both of the thyroid glands can be done to remove the tumor mass. This is usually a short procedure, but with the heart rate so high, we like to have them on methmazole for a while to get the rate down. At Animal Hospital of Montgomery we do the thyroidectomy with a CO2 laser so that there is very little pain or swelling after the surgery. Many cats regain their weight in about 30 days after surgery.

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