Hyperthyroidism in Cats

Updated: 5/31/23
A disheveled cat


Hyperthyroidism occurs when the thyroid glands in a cat’s neck produce too much of the thyroid hormone thyroxine (T4). Although the cause of hyperthyroidism is unknown, nearly all cases develop when a tumor forms on a cat’s thyroid. In very rare cases, this tumor is malignant (cancerous), but most of the time it is benign (non-cancerous). Hyperthyroidism is most often seen in middle-aged to older cats.

Risk Factors

The cause of feline hyperthyroidism is unknown, but advancing age seems to play a role.


Signs of hyperthyroidism include:

  • Weight loss
  • Increased appetite
  • Increased thirst
  • Increased urination
  • Hyperexcitability
  • Vomiting
  • Diarrhea
  • Excessive grooming

These signs develop gradually and initially may go unnoticed. For this reason, it’s important to take your cat for regular veterinary visits.


The diagnosis of hyperthyroidism is based on a combination of the clinical signs outlined above, an enlarged thyroid gland on examination, exclusion of other diseases associated with these clinical signs and an elevated level of thyroxine in the blood   If the elevation of thyroxine is borderline, additional testing may be performed to confirm the diagnosis. If the levels are high, your veterinarian will discuss treatment plans.


Hyperthyroidism can be treated in four different ways:

  • Medication: The drug methimazole is an effective and relatively inexpensive treatment. However, it will not cure the disease.  Your cat will require one to three doses of medication per day for the duration of her life.  Methimazole, is available in pill form and also can be specially compounded into a formulation that is put onto the ear flap.  Methimazole is generally very safe but occasionally causes anemia and a skin rash.
  • Radioactive iodine therapy: This treatment involves injecting your cat with radioactive iodine, which destroys the overactive thyroid tissue. This treatment causes little to no discomfort in your cat. Since radioactive materials are used, this procedure must be performed by a specialist and your cat will need to remain in the clinic for several days following the treatment because of the regulations governing radioactivity.
  • Surgery: Removing your cat’s overactive thyroid gland is a relatively easy procedure., Removal of the thyroid gland is an alternative treatment  for cats that do not tolerate methimazole.
  • Dietary therapy: The thyroid gland requires iodine to synthesize thyroxine.  Removing iodine from your cat’s diet will decrease the production of thyroxine and resolve the hyperthyroid condition. To be effective, the iodine-deficient diet must be the cat’s exclusive diet for the rest of your cats life.  Some cats will not eat this food and, in a multi-cat household, feeding cats different foods can be a challenge


Unfortunately, there are no known methods for preventing hyperthyroidism. The best prevention is early detection, so be sure to know the signs and schedule regular veterinary visits for your cat.

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Internal Medicine

About Internal Medicine at AMC AMC’s specialists in Internal Medicine are certified by the American College of Veterinary Internal Medicine. Areas of this specialty service include: Endocrinology Gastroenterology Hematology Immune

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