Hyperthyroidism is a disease caused by an overactive thyroid gland, an organ found on either side of the windpipe at the base of the neck. This gland produces thyroid hormone which helps to regulate your cat’s metabolism, or rate of bodily activity. 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 now be successfully treated.

Hyperthyroidism was first seen in cats as recently as 30 years ago and 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. The disease is rare in young cats but becomes more common in later life. It is now the most frequent hormonal disease in middle-aged and older cats.

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 symptoms are unusual and the opposite of what might be expected, such as depression, loss of appetite and physical weakness.

Apart from recognising the symptoms, there are a number of other steps in making a diagnosis. When your vet examines your cat’s throat the thyroid gland may feel lumpy or enlarged. Blood tests are usually taken to rule out other diseases of the liver or kidneys. Directly measuring levels of hormone in the blood may help confirm the diagnosis but in some cats the thyroxine levels may be normal. Your vet will also want to check your cat’s heart – an abnormally fast or irregular heart beat is often a feature. Early diagnosis and treatment is important to prevent and even reverse damage to the heart and kidneys.


There are drugs available which 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
  • Suitable for cats with severe kidney disease, which might be made worse by the other types of treatment


  • Does not tackle the underlying problem and so treatment must continue throughout your cat’s life
  • Difficulties in getting your cat to swallow tablets
  • You must remember top 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 is right


The abnormal gland can be surgically removed.


  • Treatment should permanently cure the disease so no need for further medication


  • Not suitable for all cats, such as those with severe kidney disease or the very elderly
  • Your cat may need drug treatment for a few weeks beforehand to show that its kidneys will cope and to stabilise their condition before anaesthesia
  • Needs a general anaesthetic which is always a slight risk but more so in ill animals
  • Possibility of damaging the parathyroid glands, which lie close to the thyroid and control the use of calcium in the body, so needs an experienced surgeon
  • 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


An injection of radioactive iodine will destroy the abnormal thyroid tissue while leaving normal cells unaffected.


  • No anaesthetic required and very few unwanted side-effects
  • A single treatment will permanently cure the disease in 9 out of 10 cases and a second treatment will do the trick in most of the rest
  • 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 is normally missed during surgery


  • Availability – there are only a few places offering the treatment because of tight regulations covering the use of radioactive substances and there is likely to be a waiting list
  • Your cat will have to stay in complete isolation until the radiation level has died down, usually around four weeks
  • Your cat cannot be handled during this time and so this method is unsuitable for cats needing urgent treatment for other serious conditions
  • The cost of treatment and prolonged boarding can be high

The decision on which method to choose should be made after careful discussion with your vet. Each has advantages and disadvantages and not all may be suitable for your cat. There are a number of things to consider, 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.