What is hyperthyroidism?

Hyperthyroidism is the most common endocrine (hormonal) disorder that affects cats. It is rarely seen in cats less than eight years of age and there is no sex or breed predisposition. It is caused by an excessive production of thyroid hormone by the thyroid glands situated in the neck.

Thyroid hormone controls the body’s metabolic rate, or activity level, and too much of it (hyperthyroidism) causes your cat to be hyperactive and to lose weight even if he is eating well.

What causes hyperthyroidism?

Usually, hyperthyroidism is caused either by excessive growth (hyperplasia) of the thyroid gland, or by a benign tumour of the thyroid (adenoma). Both thyroid glands are usually involved, although one gland may be more severely affected than the other. The cause of this benign change is unknown. However, it is likely that multiple factors are involved, including genetics, diet, environment and the immune system.

In rare cases, the disease may be due to a malignant tumour (thyroid adenocarcinoma).

What are the signs and symptoms of hyperthyroidism?

Clinical signs of hyperthyroidism are usually subtle at first, and get more severe as the disease progresses. The most characteristic signs are:

  • Weight loss
  • Increased appetite
  • Hyperactivity and restlessness

Less common signs include:

  • Increased heart rate (tachycardia)
  • Increased urination and/or thirst
  • Vomiting or diarrhoea

In advanced cases, your cat could become lethargic and weak and lose her appetite.

Are there any complications of hyperthyroidism?

Yes.  Cats with untreated hyperthyroidism can develop hypertrophic cardiomyopathy (in which the heart muscle becomes abnormally thickened) and hypertension (high blood pressure). Initially, these complications may require specific treatment, but usually resolve once the underlying hyperthyroidism is well controlled.

How is hyperthyroidism diagnosed?

Once hyperthyroidism is suspected, a thorough physical examination and some blood tests will be required to confirm the diagnosis.

On physical examination, an enlarged thyroid gland may be felt as a small, soft mass, on either side of the neck. It may be difficult to detect, as the glands are freely movable and can slide along and behind the windpipe (trachea). In some cats, it may not be able to be palpated.

Diagnosis is confirmed by determination of serum thyroid hormone levels. In most affected cats, both thyroxine (T4) and triiodothyronine (T3) are elevated. However, elevation of T4 is a more reliable indicator of hyperthyroidism.

In rare cases, a cat may have normal thyroid hormones, but has all the symptoms of hyperthyroidism. In those cases, your vet may do further testing. Usually all that is required is to repeat the thyroid hormone testing at another time.

Other laboratory tests that might be ordered are:

  • Liver enzymes, which may be elevated with hyperthyroidism
  • Chest x-ray or ultrasound if cardiomyopathy is suspected
  • Doppler blood pressure monitoring to check for high blood pressure
  • Testing for renal failure before starting treatment.

How is hyperthyroidism treated?

There are three options available for the treatment of hyperthyroidism: administration of anti-thyroid drug therapy, surgical thyroidectomy and radioactive iodine therapy.

Anti-thyroid drug therapy

Anti-thyroid drugs act by interfering with the production of thyroid hormone. These drugs control hyperthyroidism, but do not cure it.  Your cat will need treatment for the rest of his life.

Most of the time you will need to give your cat the medication three times a day for two or three weeks, until her thyroid level is normal. After that, she will need the medication twice a day. She will also need regular follow-up appointments with the vet.

Some cats experience mild and usually transient side effects from the medication. They usually go away within a few weeks. You could see:

  • Poor appetite
  • Vomiting
  • Lethargy

More serious side effects include low white blood cell count, clotting problems or liver disorders. Your vet will perform routine blood tests to monitor for these problems.

Surgical thyroidectomy

Surgical removal of the affected thyroid gland (thyroidectomy) can produce a permanent cure. However, in some cats, signs of hyperthyroidism may recur, due to the production of thyroid hormone at other sites previously unaffected.

Surgical skills and experience are necessary to minimise the risk of post-surgical complications.

The major risk associated with thyroidectomy is inadvertent damage to the parathyroid glands. They lie just next to the thyroid glands and are essential in regulating blood calcium levels.

If your cat has hypertrophic cardiomyopathy because of the hyperthyroidism, using general anaesthesia is a concern.  To reduce the risk of anaesthetic complications, these cats are usually stabilised with medications for a few weeks prior to surgery.

Radioactive iodine therapy

Radioactive iodine therapy (I131) can be used to provide a safe and effective cure for hyperthyroidism. Radioactive iodine is selectively taken up by active thyroid tissue and destroys the tissue.  Radioactive iodine therapy is a good option when surgery is contraindicated, or when the excess thyroid tissue can’t be surgically removed.  A single dose is curative in the majority (98%) of cats.

Radioactive iodine is the only effective treatment for cats with thyroid adenocarcinoma.

The treatment can only be performed in a specially licensed facility. Your cat will usually have to stay in the hospital until the radiation level has fallen to within acceptable limits. This is usually for a period of 7 days following treatment.


Hyperthyroidism is a common disorder of older cats and is due to an increased production of thyroid hormone. Due to the action of thyroid hormone on multiple body organs, it can have wide and varied effects. However, if recognised early and treated appropriately, the outlook for affected cats is generally good.

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