Hyperthyroidism in cats



Tumours of the thyroid glands are common in older cats. They usually over-produce thyroid hormone which increases the rate at which the body's organs work.

Symptoms vary but include: increased appetite with weight loss, poor coat sometimes with colour changes, increased heart rate, breathing problems, diarrhoea, occasional vomiting, bouts of hyperactivity and/or "depression". Eventually the cat becomes lethargic and inappetant, making diagnosis difficult.

In the early stages the changes are reversible, later they are not and so it is important to diagnose the condition as early as possible.



If the glands are enlarged and the symptoms are typical then diagnosis is easy. Unfortunately this is not always the case so we sometimes have to look for increased levels of thyroid hormone in the blood. Unfortunately blood levels vary during the day and also are reduced if the animal is unwell for any reason, which may mean several blood tests to confirm the diagnosis.



The symptoms are caused by the increased levels of thyroid hormone and so treatment is aimed at reducing these levels.

This can be done medically with tablets that stop the thyroid gland releasing thyroid hormone or using radioactive iodine to destroy the thyroid producing cells, or surgically by removing the diseased gland or glands. There is a new treatment available using a special food that has a very low level of Iodine in it, which stops the body making thyoid hormone.

Medical Treatment. The aim is to reduce the level of circulating thyroid hormone to normal. Because some cats produce more hormone than others, the amount of drug needed daily will vary from one cat to another.  We will need to monitor the cat's condition closely and the medication will need to be continued for the rest of the cat's life because if the drug is stopped the thyroid hormone level will rise again. There are two drugs licenced for use in cats. They both come in tablet form and one as a gel that you put on the skin, usually inside your cat's ear! The gel is very easy to use but quite expensive.

Hills y/d, if used as the sole food, will stop the thyoid gland making thyroid hormone, so controlling the problem. However your cat will have to only have the special food for the rest of it’s life. Most cats like the food, but some do not. It is important to chnage from their old food very slowly, over at least 6 weeks.

Radioactive Iodine can be used to destroy the affected thyroid glands. This is the treatment of choice however the cat then has to be hospitalised for a month at a referral centre and it is expensive. The advantages are that there are very few side effects, the cat then does not need long term mediation or special food and the conditions does not spread elsewhere in the body.

Surgical Treatment. The affected thyroid gland(s) can be surgically removed. If only one gland is involved this is usually a relatively simple and painless operation with few post-operative complications and means that the cat no longer needs routine medication. In most cases the cat can go home later that day with no need for further medication other than antibiotics for a few days. Most make a full recovery within a few days, eating less, putting on weight and becoming much more settled.

However in the long term other growths can occur usually in the other thyroid gland initially, but this usually takes several years.

If both thyroid glands have to be removed at the same time there is a danger that the blood supply to the parathyroid glands, which regulate the blood calcium level, can be affected and so we routinely hospitalise them and monitor their blood calcium for two to three days after the operation so we can start them on treatment if there are any problems.

Even in cats that need calcium supplementation, this is often only temporary.

Unfortunately hyperthyroidism makes cats a greater anaesthetic risk than normal and so we often recommend a course of tablets before the operation and pre-operative blood tests so we can ensure that the anaesthetic is as low risk as possible.

Cats rarely need thyroid tablets even if both glands are removed, probably because there is other thyroid hormone producing tissue in the chest.

Surgery is therefore the usual treatment in younger, fit cats or for cats that are not happy to be given tablets daily for the rest of their lives!



Hyperthyroidism is the most common hormonal disease in cat. Left untreated it causes premature aging and death, however the condition usually responds well to treatment and if caught early there is a good chance of a complete recovery.

The treatment we recommend varies enormously depending on many factors and so each case is assessed individually.