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Feline Hyperthyroidism

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What is Feline Hyperthyroidism?

Feline hyperthyroidism is a common condition in aging cats. Usually affecting cats older than thirteen years of age, it can be seen in cats as young as five years of age. There does not appear to be any individual breed or sex that is especially at risk. Hyperthyroidism can be seen in dogs, although it is rare.

What are the symptoms and diagnoses of hyperthyroidism?

The symptoms of hyperthyroidism are the result of the overproduction of thyroid hormone by a tumor of the thyroid glands that are located in the neck. This hormone, (T4, also called thyroxine), helps maintain appropriate metabolism, so when an excess is released, the metabolism speeds up substantially. One or both lobes of the thyroid gland can be affected.
The signs include:Location of the thyroid gland

  • rapid weight loss
  • increased appetite, (some cats become ravenous!)
  • increased drinking
  • urination
  • and some behavior changes
On physical exam, the doctors will be able to detect a very rapid heart rate and sometimes can feel enlarged thyroid glands. A blood test will be done to confirm the presence of excessive thyroid hormone (T4).

The overproduction and release of thyroid hormone stimulates the heart to pump faster and more forcefully causing an increase in blood pressure. The heart will eventually enlarge to meet the demand, which will cause damage to the heart and kidneys. The over stimulation can also cause extreme weight loss, so immediate detection and treatment is advised. Annual blood screening on cats over ten years of age is advised to detect hyperthyroidism before damage has occurred.

If an elevation in the hormone is found, treatment with an oral medication, surgery, or radioiodine is advised. The medication, tapazole, is used to regulate the thyroid hormone production. Because this medication is given twice a day and because some cats will develop an allergy to the medication - surgery or radioiodine therapy is advised after the levels of thyroid hormone are normalized.

Removal of the thyroid glands reduces the production of hormone, allowing the cat’s metabolism to return to normal. Rarely, cats may require supplements after surgery.

Who gets hyperthyroidism?

Primarily cats, ages 8 and over are affected. It has been reported in cats as young as 5 years of age. Rarely, dogs are diagnosed. In dogs, it is usually a malignant tumor of the thyroid rather than an adenoma, as it is in cats.

What is the treatment for hyperthyroidism?

In lieu of surgery and medication, radioiodine therapy can be performed. While this procedure does not require anesthesia, hospitalization for ten days to two weeks in an approved facility is required. The closest location to Peoria for this is the University of Illinois.

The doctors of Whitney Veterinary Hospital currently recommend three weeks of tapazol prior to surgery. Surgery is performed on an outpatient basis. The patient will go home the same day surgery is performed and return in ten days to have sutures (stitches) removed. Rarely is any follow-up therapy needed.

In 5 - 10% of cases, recurrence of the thyroid tissue is possible within five years. This is unusual; however, re-operation to remove the growth will correct the problem at that time.

Risks associated with surgery are generally minor, however because the thyroid gland is tightly attached to the parathyroid gland, it is possible to damage the parathyroid during surgery. If this happens, temporary or permanent calcium imbalances can occur. This can be life-threatening. Your veterinarian will discuss the risks and the outcomes of these procedures with you in detail.

Are there any complicating factors?

  • If left untreated, this disease can lead to congestive heart failure, untreatable diarrhea, eye damage, retinal detachment (as a result of hypertension), extreme weight loss, and death.
  • The prognosis is less favorable for cats with underlying renal disease. (It can be secondary to chronic hypertension or unrelated to the thyroid disease), .
  • If renal disease is suspected in a cat with hyperthyroidism, a reversible form of treatment - perhaps anti-thyroid drugs - will be recommended.

If you have any questions about hyperthyroidism please feel free to call Whitney Veterinary Hospital at
We’ll be happy to answer any and all of your questions.


  • Tilley, LP, Smith, FWK, The 5 Minute Veterinary Consult, Canine and Feline. 1997, Williams & Wilkins.
  • Bonagura, JD (ed.) Kirk's Current Veterinary Therapy XIII Small Animal Practice. 2000,W. B Saunders Company.

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