Hyperthyroidism in Cats

About Hyperthyroidism

Hyperthyroidism was not recognised as a problem in cats until 1979. It is now considered a common disease of cats. Both male and female cats are equally affected. It is thought that Siamese and Himalayan cats have a decreased risk of developing hyperthyroidism.

What is the thyroid gland and what does it do?


Cat thyroid gland.The thyroid gland is divided into two lobes, one on either side of the windpipe in your cat’s neck. The thyroid gland produces thyroid hormone (thyroxine). In normal cats the gland cannot be felt.

The hormone produced by the thyroid gland is essential for the normal growth of the skeleton and brain in young animals. It also has a wide variety of functions in adult animals:
→ Involved in the control of metabolism
→ Effects heart rate
→ Helps control the breakdown of fatty tissues
→ Involved in red blood cell production

What is hyperthyroidism?

Hyperthyroidism is when the thyroid gland makes and secretes very high levels of thyroid hormones. This is caused by abnormal changes or tumors in the gland. These abnormalities are usually benign and can be treated successfully. In rare cases, hyperthyroidism can be more complicated to treat. The cause of the abnormal changes in the thyroid tissue is not known.

Signs of Hyperthyroidism.


Cat with hyperthyroidism symptoms infographic.

Hyperthyroidism is a progressive disease with a slow, subtle onset, becoming more obvious with time. Early signs may be hard to recognise because the increased appetite and high levels of activity often seen are not always recognised as abnormal. The gradual deterioration in coat and body condition can also be wrongly attributed to the “normal signs” of aging.

Thyroid hormones control the speed of your cat’s metabolism, but when in excess, can cause a range of signs. The more thyroid hormone produced, the higher the metabolic rate and the more calories your cat burns.

Not all of these signs will occur in every cat with hyperthyroidism.

Poor body condition is a common sign of hyperthyroidism.
Hyperthyroid cats might be nervous or aggressive.
An enlarged thyroid gland is sometimes visible but more often can be felt.

Diagnosis of Hyperthyroidism

Your vet will diagnose that your cat is hyperthyroid from the history that you give, a thorough physical examination and laboratory tests.

Conditions such as kidney disease and heart problems are also common in older cats. It is important to check for concurrent diseases, this can influence the choice of treatment and prognosis for your cat.

To confirm the diagnosis of hyperthyroidism your vet will take blood samples to measure the levels of thyroid hormone circulating in the bloodstream and to evaluate your cat’s general condition and make sure it is not suffering from any other diseases.

In some cases, even although your vet strongly suspects that your cat is hyperthyroid, as the tests may not be conclusive.

This can be caused by a variety of factors:

  • Your cat may be at a very early stage of the disease
  • Thyroid hormone levels can fluctuate and can even be normal at some point in hyperthyroid cats.
  •  Other diseases can influence thyroid hormone levels.
  • Your vet may need to repeat the blood tests after a week or so.

In some cases in might be necessary to carry out additional tests as well.

These tests could include:

  • Special tests to evaluate thyroid gland function
  • Diagnostic imaging (e.g. nuclear scintigraphy, ultrasound) of the thyroid, particularly before radio-iodine therapy or corrective surgery is done.

Treating Hyperthyroid Cats

Hyperthyroidism is usually manageable and there is a good chance that your cat will return to normal. The aim of treatment is to reduce the level of and ultimately the effects of excessive thyroid hormone.

Cat meets hedgehog.

It is very important that cats diagnosed with hyperthyroidism are treated as soon as possible. The longer a cat is left untreated the more detrimental the effects of the excessive thyroid hormones. Your vet will discuss your pet’s treatment plan with you and together you can decide on the best option for your cat.

The most common treatments options are:

• Medical treatment: Medical treatment is used for long term management of hyperthyroid cats and prior to radioactive iodine treatment or surgery. Drugs that block the manufacture of the thyroid hormones are used to reduce the levels of these hormones back to normal.

• Radioactive Iodine: Radioactive iodine treatment requires specialised facilities and hospitalisation. It is the most effective treatment in cases with malignant thyroid tumors. It can however result in hypothyroidism – which may require supplementation of thyroid hormones.

• Surgery: Surgical thyroidectomy involves the removal of one or both lobes of the thyroid gland. Cats usually undergo 2-4 weeks of medical treatment to improve their condition and minimise the potential complications, such as heart irregularities.

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