Hyperthyroidism (Hyperactive Thyroid Gland) in Cats

Hyperthyroidism (Hyperactive Thyroid Gland) in Cats

Animals affected

Middle-aged and elderly cats.


Hyperthyroidism is a disease of the thyroid gland. Located in the neck, the thyroid gland produces a hormone called thyroid hormone. Although thyroid hormone plays a complex role in the body, its main effect is to increase the rate of metabolism. Hyperthyroidism causes excessive production of thyroid hormone. Therefore, the main problem in hyperthyroid animals is increased metabolism.


Symptoms of hyperthyroidism include:

Risk Factors

Age is the primary risk factor.  Additionally, hyperthyroidism is more common in cats whose diets contain canned food than in cats that eat only dry food.


Untreated hyperthyroidism can lead to emaciation, heart disease, blindness, high blood pressure, and kidney failure.


Basic blood and urine tests provide a definitive diagnosis of hyperthyroidism in most cases.


Several effective treatment options reduce the incidence of complications.

  • Medication: Methimazole (Tapazole®) temporarily inhibits the thyroid gland’s production of thyroid hormone. Lifelong medication generally is required, unless radioiodine therapy or surgery are performed. Each cat’s response to methimazole is unique, which means that cats on the medication may require several rounds of blood tests and medication adjustments before the proper dose is established.
  • Radioiodine therapy: Hyperthyroidism can frequently be cured with radioiodine therapy. A special type of iodine is injected into the cat, which permanently (in most cases) returns the thyroid gland to a normal state. The procedure is noninvasive and not painful, but several days of hospitalization are required as the cat clears the special iodine from its system.
  • Surgery: In some cases, surgical removal of the thyroid gland can permanently cure hyperthyroidism. For several reasons, most veterinarians feel that surgery is an inferior option to radioiodine therapy.
  • Diet: A prescription diet (y/d, manufactured by Hill’s) to treat hyperthyroidism was released in 2011.  Many experts remain unconvinced of its efficacy and safety, particularly in cats that suffer from both hyperthyroidsm combined with another health problem such as kidney disease.  y/d may be a good choice in cats for whom other treatmens are not possible, but most experts don’t feel it is ideal as a first choice treatment for hyperthyroidism.


Hyperthyroid cats require regular blood tests to assess the efficacy of treatment. In animals that have not undergone radioiodine therapy or surgery, the disease may progress and worsen. Therefore medication adjustments may be necessary at intervals of three to six  months.

Regular heart and eye evaluations and blood pressure monitoring may be necessary in some cats.


Hyperthyroidism in cats is similar to Grave’s disease in people.

Hyperthyroidism is almost unheard of in dogs.

Copyright © Eric Barchas, DVM All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.