Pet Factsheets

Hyperthyroidism - disease and treatment

The thyroid glands are small organs found just under the skin, either side of the windpipe in the neck. These glands produce thyroid hormone which helps to regulate your cat's metabolism, or rate of bodily activity. Production of thyroid hormone is normally closely regulated in the body as the effects of too much (or too little) can be very serious. Hyperthyroidism is a disease caused by overactivity of these glands. When the thyroid gland produces too much hormone, your cat's 'internal motor' effectively goes into overdrive. Untreated this would eventually be fatal but the condition can be successfully treated.

What causes the disease?

Hyperthyroidism appears to be caused by a form of benign cancer in the thyroid gland. However, it is still not clear what causes the cancer to develop. In affected cats the thyroid gland increases in size and starts to produce thyroid hormone all the time without taking heed of the body's normal regulatory messages. In seven out of ten cats both glands are affected. The disease is rare in young cats (less than 7 years old) but becomes more common in later life. It is now the most common hormonal disease in middle-aged and older cats.

What other conditions are associated with hyperthyroidism?

If untreated the effects of excess thyroid hormone are seen in many organs. Heart rate is increased and changes occur in the muscle of the heart making it thickened and stiff. This can lead to signs of heart failure, irregular heart rhythms and blood clots may form in the circulation which can damage other organs.

High blood pressure is another side effect of the high thyroid hormone levels. Cats with high blood pressure may suffer damage to many other organs particularly the eyes (causing blindness), kidneys and brain.

Cats with hyperthyroidism also often have kidney disease - it may be that the two conditions are both just common in the older cats. However, the high blood pressure in cats with hyperthyroidism may actually help to support kidney function and when treatment for hyperthyroidism is instituted the drop in blood pressure may cause kidney problems to get worse.

How would I know if my cat has hyperthyroidism?

The first indication that anything is wrong is usually a marked increase in your cat's appetite. Even though your cat is eating more it may lose weight and its coat may become rough and unkempt. Other changes include restlessness and aggression, body tremors, increased drinking and urinating, vomiting and diarrhoea. In about one case in ten the signs are not typical and can include depression, loss of appetite and physical weakness.

How can my vet diagnose hyperthyroidism?

Apart from recognising the disease from your description, there are a number of other steps your vet can take to make a diagnosis. When your vet examines your cat's throat the thyroid gland may feel lumpy or enlarged. Your vet will also want to check your cat's heart - an abnormally fast or irregular heart beat is often a feature of the disease and may indicate changes in the heart muscle caused by the high hormone levels. If your vet is concerned about your cat's heart they may take an x-ray, perform an ultrasound examination or an ECG. Blood pressure can be measured in the clinic or at home, and a high blood pressure may be an indicator of hyperthyroidism.

Blood tests are usually taken to rule out other diseases of the liver or kidneys. Thyroid hormone levels can be measured in the blood but in a few cats the thyroxine levels may be normal although your cat has the disease. Early diagnosis and treatment is important to prevent, and even reverse, damage to the heart and kidneys.

What treatments are available?


Some drugs block the production of hormones by the thyroid gland. The medication is given one to three times a day.

  • Simple and does not require an anaesthetic.
  • Side effects are not common (particularlay after the first few weeks of treatment).
  • Suitable for cats with severe kidney disease which might be made worse by the other types of treatment and is generally used before other types of irreversible treatment to monitor the effect of reducing hormonal levels on kidney function.
  • Does not tackle the underlying problem and so treatment must continue throughout your cat's life.
  • The temperament of some cats is not suitable for daily medication. Cats may become very distressed when you try to administer tablets and this has to be done every day for the rest of your cat's life.
  • You must remember to give the tablets every day.
  • In some cats there are side effects of the drug ranging from fatigue to anaemia.
  • In the early stages your cat must be carefully monitored to make sure that the dose of drug is right.


The abnormal gland can be surgically removed.

  • This avoids the need for regular medication, because once the gland is removed no more hormone can be produced.
  • Not suitable for all cats, such as those with severe kidney disease or the very elderly because of the risk of anaesthetic.
  • Needs a general anaesthetic which always carries a slight risk but more so in sick animals.
  • Your cat may need drug treatment for a few weeks before surgery to stabilise their condition before anaesthesia. The drug treatment will also give an idea of how your cat will be after surgery. 
  • Possibility of damaging the parathyroid glands, which lie close to the thyroid and control the use of calcium in the body, so surgery must be done by an experienced surgeon.
  • In a few cases a bit of the abnormal thyroid tissue is growing separate to the main gland and may be missed at surgery.
  • After surgery cats should be carefully monitored for a couple of weeks to make sure there are no changes in blood calcium caused by parathyroid gland damage.


This involves an injection of radioactive iodine to destroy the abnormal thyroid tissue while leaving normal cells unaffected.

  • No anaesthetic required as the dose is given by injection under the skin. 
  • Very few unwanted side-effects.
  • A single treatment will permanently cure the disease in 9 out of 10 cases and most of the rest will be cured by a second treatment.
  • Radiation will also work in much rarer cases in which the tumour is malignant or where a portion of thyroid tissue has broken away from the main gland and therefore would not be removed during standard surgery.
  • Availability - treatment cannot be offered at all veterinary practices because of tight regulations covering the use of radioactive substances and there may to be a waiting list at specialist centres.
  • Your cat will have to stay in complete isolation until the radiation level has died down, usually around four weeks and so will have to be boarded at the facility.
  • Your cat cannot be handled during this time and so this method is unsuitable for cats needing regular treatment for other serious conditions.
  • The cost of treatment and prolonged boarding can be high.
  • Blood tests are needed at intervals to ensure that thyroid hormone levels remain controlled in the long term.

Dietary treatment

Since all iodine required to make the thyroid hormones comes from the diet, it is possible to control how much thyroid hormone a cat can make by controlling their dietary intake of iodine. Special diets with restricted iodine are available and cats fed these diets can only make a normal amount of thyroid hormone.

  • Dietary food is available from vets and is easy to administer at home.
  • Effective in most cases with no side effects.
  • Cats must be fed the special diet exclusively - if they get access to other food, eg by hunting or visiting other homes then they will get extra iodine and the food will be ineffective.
  • Requires continued regular monitoring of thyroid hormone levels to ensure disease is controlled.

How should my cat be managed after treatment?

Whatever treatment has been used it is important that regular monitoring is continued. If your cat is on tablets to control their disease they may also need regular monitoring of thyroid hormonal tablets to ensure the tablets are having maximal effect.

There may be many changes caused by the high hormonal levels such as heart changes which will reverse over time after treatment. Sometimes permanent damage will have been done before treatment, eg if your cat is blind that will not resolve after treatment, and so there may be long term management for other conditions. Kidney function should be measured regularly immediately after treatment to make sure that function does not deteriorate as hormonal levels drop.

What is best for my cat?

After treatment, signs of hyperthyroidism usually resolve in almost all cats. The decision on which method to choose should be made after careful discussion with your vet. Each method has advantages and disadvantages and not all may be suitable for your cat. There are a number of things to consider before selecting a treatment: your cat's age, the severity of the condition, the presence or absence of other diseases and the risk of complications, etc. Cost may also be a factor as both surgery and radiation treatment can involve a significant expense. However, medication may also be costly in a cat diagnosed with the disease relatively early in its life and treated continuously for several years.

Hyperthyroidism, although a serious disease, can be cured with no long-lasting effects for your cat if appropriate treatment is provided early. If you are worried about any signs in your cat always seek veterinary advice.