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A Guide To Feline Hyperthyroidism

Aimee Labbate
  • Aimee Labbate

  • RCVS: 700039


What is hyperthyroidism?

Hyperthyroidism is seen quite commonly in older cats and is usually relatively easy to treat. The condition results from an overproduction of thyroid hormone from the thyroid glands in the neck. Given that the thyroid hormones play a major role in controlling metabolism (as well as some other important jobs), higher levels than normal cause the metabolism to speed up. This can have a negative effect on various organs around the body, such as the heart, leading to heart disease in some cases. Hyperthyroidism can also lead to high blood pressure (hypertension) which can cause damage to several organs including the eyes, kidneys, brain and heart. It is therefore important that, once diagnosed, hyperthyroidism is treated.

Which cats are more vulnerable to hyperthyroidism?

Hyperthyroidism typically affects middle-aged and older cats and is very rare in cats under 7 years of age.

What causes hyperthyroidism in cats?

In the vast majority of cases hyperthyroidism is caused by benign (non-cancerous) change in the thyroid glands in the neck. The underlying cause of this change is currently unknown. In very rare cases the change in the thyroid gland can be a malignant (cancerous) tumour.

What are the symptoms hyperthyroidism in cats?

The classic symptoms of hyperthyroidism include:

  • Weight loss despite an increased appetite (can be ravenous)
  • Increased thirst
  • Increased activity, restlessness, can be irritable
  • High heart rate
  • Poor and unkempt coat
  • Vomiting and diarrhoea may be present
  • High blood pressure

While most hyperthyroid cats have a good or increased appetite, some cats will present less typically with a loss of appetite, lethargy and weakness.

How is hyperthyroidism in cats diagnosed?

A diagnosis of hyperthyroidism is often suspected from the clinical signs and physical examination alone, but in order to definitively diagnose it blood tests are run to check the level of thyroid hormone in your cat’s blood.

If your cat is diagnosed with hyperthyroidism then it is normally recommended that they undergo some other tests to check their blood pressure and heart for instance. The high thyroid levels can cause the heart to work harder than it should, which can cause a thickening of the wall of the heart (hypertrophy) – this can be diagnosed using an ultrasound of the heart (echocardiogram).

Once treatment is started, the kidney function is monitored closely, as hyperthyroidism can mask kidney disease. This means that, once the hyperthyroidism is treated, pre-existing kidney disease can reveal itself and therefore needs to be managed.

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How can hyperthyroidism in cats be treated?

There are various different treatment options available for hyperthyroidism, so it is important to have a careful discussion with your vet to decide which treatment is most suitable for them.

Medical management

Hyperthyroidism can be treated with daily medication to suppress the thyroid levels – this can be in tablet or liquid form. All cats are given a starting dose and this is then adjusted according to repeated blood tests to ensure sufficient medication is given to suppress the thyroid hormone level back to normal. These medications need to be given life-long and regular monitoring is needed to ensure the thyroid level remains suppressed. They are not curative treatments.


A common treatment for hyperthyroidism is the surgical remove of the abnormal thyroid glands(s) in the neck. Cats are initially stabilised on medication prior to surgery, to monitor how they respond to treatment. The thyroid glands are inspected at surgery, and the abnormal gland(s) are removed. If only one is removed then there is less chance of post-operative complications than if both are removed, but there is the chance that the remaining thyroid gland may also develop the same abnormal changes in due course, and therefore need removing at a later date. Your cat will undergo post-operative blood tests to ensure that the calcium balance is not affected and that the thyroid level does not drop too low. In rare cases if the thyroid level drops very low then the cat may need to be given synthetic thyroid hormone tablets.

Radioactive Iodine

This is probably the safest and most effective treatment option. The cat is given a single injection of radioactive iodine (I-131) under the skin. The iodine is only absorbed by the abnormally functioning thyroid glands and does not affect any other organ in the body. It becomes concentrated in the thyroid glands and destroys the hyper-functioning (abnormal) thyroid tissue. It is a curative treatment in 95% of cases and there are no significant side-effects. This treatment is only available in specialist veterinary centres, and because the cat is temporarily radioactive following the injection, they must be hospitalised for a short period of time (typically 3-6 weeks).

Dietary treatment

With this treatment the cat is fed an iodine-restricted diet such as Hill’s Y/D. Since iodine is used by the thyroid glands to make the thyroid hormones, if there is less iodine available then they are unable to produce such large amounts of thyroid hormone, causing levels to drop to normal. While this treatment can be effective for some cats, it is crucial that they are not fed anything else except for the specialised diet, which can be difficult in outdoor cats. This specialised diet (Hill’s Y/D) is available to buy in our shop – please follow this link.

What is the prognosis of hyperthyroidism in cats?

Most cats can be treated successfully, but it is important to find the treatment regime that is most appropriate for your cat. Some treatments are curative (e.g. radioactive iodine), whereas others need to be continued life-long (e.g. medical management). Each treatment option has its own advantages and disadvantages, so it is important that you discuss these with your vet before deciding on which to use.

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