Hyperthyroidism is a condition that commonly occurs in older cats. The thyroid plays an important role in maintaining the body’s metabolism. Hyperthyroidism is an excess of thyroid tissue that can be present as a benign growth on the thyroid gland itself of may be scattered along the neck region. In 99% of cats, these tumors are benign and result in excess production of thyroid hormone. The increase in the amount of thyroid hormone circulation in the bloodstream speeds up the amount of energy used by cells in the body and results in the presenting clinical signs.

Clinical Signs

  • weight loss along with ravenous appetite
  • increased thirst and urination
  • increased heart rate
  • increased activity in some cats


Diagnosis is based on clinical sights, physical exam, and bloodwork. Bloodwork shows an elevated T4 level, the active form of the hormone.


Hyperthyroidism requires life long treatment. The abnormal thyroid tissue will continue to grow requiring an increase in the dose of medication over time. There are several treatment options for hyperthyroidism. Starting your cat on medication to reduce the extra thyroid hormone in the bloodstream is the first stage. Tapazole (methiamazole), the medication most commonly used to treat hyperthyroidism, is available in oral tablet form, flavored chew, oral liquid, or as a transdermal gel applied to the inside of your cat’s ear. Rarely, a cat has side effects to methiamazole. These include vomiting and itching, primarily around the face and neck. These side effects will generally start 3 weeks after the medication is begun. If your cat exhibits these signs, you should discontinue the medication and call the clinic.

Once your cat has been on tapazole for 3-4 weeks, you will need a recheck appointment to ensure the medication is at an effective dose and to recheck the functioning of the kidneys. Because hyperthyroidism speeds up the heart rate, there is increased blood flow to the kidneys which may mask early kidney problems. Once the thyroid hormone levels are normalized, kidney function may not be optimal. Your cat may require medication to support the kidneys. Surgery is an option if only one side of the thyroid gland is enlarged and able to be felt on exam. The procedure is straight forward and cats recover well. However, there is a chance that the remaining thyroid gland will develop abnormal tissue. This would require an additional surgery.

Radioactive Iodine (I-131) is generally a one-time only injection under the skin. It targets abnormal tissue and destroys the cells. Normal thyroid tissue is not affected in the majority of cases; however, there is about a 1% chance it will affect the normal thyroid tissue.

Cats need to be stabilized on oral medication first to assess the functioning of the kidneys. The treatment does require a 5-7 day hospital stay until the radioactive levels are below a certain level. This procedure is offered at a treatment center in Los Angeles.

5% of cats receiving this treatment become hyperthyroid once again. Once your cat is discharged home, T4 levels and kidney values should be checked 1 to 2 months after surgery. If T4 levels remain normal, your cat should not need any more treatment for his/her hyperthyroidism.

Hyperthyroidism is a common, manageable condition in older cats. We can develop a specific plan that will work for both you and your cat!