Hyperthyroidism (overactive thyroid glands) is a very common disorder of older cats. It is caused by an increase in production of thyroid hormones from the thyroid glands, which are situated in the neck.

Clinical Signs

  • Weight loss
  • Increased appetite
  • Increased thirst
  • Increased irritability
  • Increased heart rate
  • Unkempt coat
  • Mild diarrhea &/or vomiting

Secondary complications

Because thyroid hormones have effects on virtually all the organs in the body, this disease can sometimes cause secondary problems.

  • Thyroid hormones stimulate a faster heart rate and a stronger contraction of the heart muscle. This means that sometimes secondary heart disease develops and additional treatment may be required to control this. Once the underlying hyperthyroidism has been controlled, the cardiac changes will often improve.
  • High blood pressure can be diagnosed along with hyperthyroidism, drugs may be needed to control the blood pressure.
  • Kidney disease does not occur as a direct effect of hyperthyroidism, but the two diseases often occur together because they are both common in older cats. Just occasionally, successful treatment of the hyperthyroidism results in a dramatic decline in kidney function.
  • Assessment of routine blood and urine tests is usually advised to help rule out any other concurrent disease (such as kidney failure).


There are three main options for the treatment of hyperthyroidism, each with advantages and disadvantages:

Medical management (drug therapy)

Thyroid hormone concentrations usually fall to within normal levels within 3 weeks. Treatment is then adjusted according to response.

Routine blood tests should be checked periodically during treatment to monitor the effectiveness of therapy and to monitor kidney function.

Surgical thyroidectomy

Surgical removal of the affected thyroid tissue (thyroidectomy) can produce a permanent cure.

However, even after successful surgery, occasionally signs of hyperthyroidism develop again if previously unaffected thyroid tissue becomes diseased.

To reduce anaesthetic and surgical complications, where possible it is always recommended that patients are initially stabilised with anti-thyroid drugs for three to four weeks before surgery.

Radioactive iodine therapy

Radioactive iodine is administered as a single injection given under the skin – the iodine is then taken up by the active (abnormal) thyroid tissue only and destroys the affected abnormal thyroid tissue.

This does involve the handling and injection of a radioactive substance which carries no significant risk for the patient, but protective measures are required for people who come into close contact with the cat. For this reason, the treatment can only be carried out in certain specially licensed facilities and the cat has to remain hospitalised until the radiation level has fallen to within acceptable limits (usually for between 3-6 weeks).

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