Cat Diseases & Illnesses – Hyperthyroidism


Overview of Hyperthyroidism in Cats

Hyperthyroidism is a very common disease in older cats. The thyroid gland is in two halves situated either side of the neck and the disease occurs when the gland secretes too much of the hormone. This is usually caused by a benign, non cancerous growth called hyperplasia and it is often only one side that is affected initially although the other side is very likely to be affected in the future.

Symptoms of Hyperthyroidism in Cats

Thyroid hormone regulates the metabolism so an excess will increase the metabolism and the symptoms you will see in your cat are mainly related to this.

You may notice your cat drinking more (polydipsia) and therefore passing more urine. You may also notice excessive hunger (polyphagia) but despite this, weight loss is a common sign of this disease. The hair coat may look in poor condition, you may notice vomiting, diarrhoea, cystitis type signs such as squatting frequently and blood in the urine and even breathing difficulties. Behavioural changes are also possible. Cats with hyperthyroidism may seem more skittish or highly strung than usual and may be restless and unwilling to settle.

If you see any of these signs in your cat then you should have them examined by a vet.

Diagnosis of Hyperthyroidism in Cats

The vet may have a suspicion that your cat has hyperthyroidism from the clinical examination and history. The thyroid gland itself is often swollen and heart rate is frequently faster than normal, often exceeding 200 beats per minute and sometimes accompanied by a heart murmur. The vet will also be able to weigh your cat and see if they are underweight or have lost weight since their last visit.

Blood tests will need to be done to achieve a definitive diagnosis of hyperthyroidism. These tests measure the amount of thyroid hormone, also known as T4 that is in your cat’s bloodstream. This is often done in conjunction with further bloods tests to check other organs such as the liver and kidneys.

If the thyroid hormone levels are shown to be high then a diagnosis of hyperthyroidism can be made and treatment started as described below. If the hormone levels are borderline but the history and clinical signs are very strongly suggestive of hyperthyroidism then there is another test that can be run on the blood which measures the thyroid hormone in a different way and this may then confirm a diagnosis of hyperthyroidism.


There are three ways of treating cats with hyperthyroidism:

1) Radioactive Iodine therapy. The thyroid gland uses iodine to make thyroid hormone and in this form of treatment, affected cats are injected with a radioactive form of iodine which, when is taken up selectively by the thyroid gland. It destroys the cells in the gland which are producing the excess hormone but other tissues in the body are not affected. 

This is performed at specialist centres where cats are hospitalised for up to a month until the radiation reduces to levels at which it is safe for the cats to come home.

There are no serious side effects from this treatment and it will cure around 95% cases. It can be a good option for cats who will not take tablets or who are not stable enough for a general anaesthetic.

2) Surgery to remove the affected half of the gland is another option. The blood vessels to the gland are tied off and the capsule around it is removed to take out the gland. After successful surgery, cats do not need long term medication but there are potential risks with this option. There is always a risk putting animals under general anaesthetic and this is increased if they are unwell. In order to minimise this risk, cats undergoing surgery are often stabilised with tablets, explained below, for two weeks or so before the procedure. Your cat will also have stitches in place after surgery which may need to be removed by the vet ten days later.

Although surgery resolves the problem in the affected gland the other half of the gland is likely to become affected in the future as previously mentioned. Repeated surgery is certainly possible but is more complicated as adjacent to the thyroid glands are the parathyroid glands which control calcium balance in the body. These can be damaged during surgery but if this happens in the first surgery then the other side can compensate. If both sides are damaged then long term medication will be needed to help control calcium levels in the body.

3) Medical treatment with daily tablets is the third option for treatment of hyperthyroidism. These tablets work by blocking the production of the thyroid hormone in the gland. There are two different drugs that are commonly used to control hyperthyroidism.

Felimazole (active ingredient is methimazole) and Vidalta (active ingredient is carbimazole).

Felimazole (carbimazole) is a small sugar coated tablet which is given twice daily with food. Felimazole comes in two different strengths, a 2.5mg tablet and a 5mg tablet, available in pots of 100 tablets. The exact dose required by your cat will be decided by your vet.

Vidalta (methimazole) is a once daily tablet, also given with food and is available in two different strengths, a 10mg tablet and a 15mg tablet. Vidalta is available either in pots of 30 tablets or pots of 100 tablets. Again the dose needed by your cat will be advised by your vet.

These tablets can occasionally have side effects such as vomiting or going off food but these are often temporary or can be improved by reducing the dose.

With either medication, regular blood tests will be needed to monitor the thyroid hormone levels in the blood to check that the medication is working and the hormone levels are returning to normal. The exact frequency of the tests will be decided by your vet.

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