Hyperthyroidism in Cats

Information about hyperthyroidism in cats, including diagnosis and treatments

Hyperthyroidism is caused by excessive amounts of thyroid hormone being produced by a tumour in a cat's thyroid glands. Cats normally have 2 thyroid glands that sit either side of their windpipe in their neck. In 80% of hyperthyroid cases one (or both) of these glands can be felt by the vet as being enlarged. Some cats have thyroid tissue inside their chests that cannot be felt on a physical examination. Hyperthyroidism is very commonly seen in middle to older aged cats. In the vast majority of cats, the tumours are benign in nature, limited only to the thyroid tissue. However, over time, some will change to become more malignant and spread to other tissues. This is one of the reasons to pick up these cases as early as possible and get them treated. The other reason is that the excessive thyroid hormone produced by these tumours have serious effects on your cats health. Left untreated these cats decline quickly and become very weak and unwell.
Thyroid hormone has lots of functions but it's easiest to think of it as setting your cat's metabolism 'tickover' rate. Too much thyroid hormone therefore drives the cat's metabolism too hard resulting in the following common symptoms:

  • Weight loss - despite a good appetite (but 5% of cats will have gone off their food)
  • Fast heart rate (sometimes with a murmur)
  • Increased vocalisation / restlessness / aggression (some cats may require sedation to blood sample)

Other signs may also be present such as:

  • Poor coat quality / hair loss
  • Vomiting / Diarrhoea
  • Neurological signs (head tilt, circling, seizures)


Although we may suspect hyperthyroidism when we examine a cat we must always confirm the diagnosis with a blood test. It's important at this point also to check that these patients have no other issues that would prevent them from receiving treatment.
Occasionally we can see suspected hyperthyroid cases with many of the appropriate symptoms of the disease but a thyroid level within the normal range. In these cases we may need to repeat the test in a month or so, or perform additional testing.


Initially we tend to start most cats on anti-thyroid medication for about 3 weeks to get them stabilised and better assess their general health. We perform a complete blood screen to look at liver and kidney function and check for any side effects of the medication. Long term management involves one of the following treatments:

  • Radioactive Iodine
  • Surgery (Thyroidectomy)
  • Medical Management
  • Iodine Restricted Diet

Radioactive Iodine

For most cats this is the 'Gold Standard' treatment for Hyperthyroidism (and is the preferred treatment for humans with the disease as well). The cat is injected with a radioactive form of iodine that is taken up by the thyroid tumour and destroys it.

Once the cat is injected it is placed in isolation for up to 2 weeks as it is a potential risk to people and other pets. For this reason cats have to be screened carefully before treating to make sure they are safe to be left in isolation for this length of time. This may involve further blood tests and potentially a heart scan.
All treated cats will become transiently hypothyroid (too little thyroid hormone) and some of these cats may require long term supplementation later in life but this is easily given, effective and not costly.
It is also possible for treated cats to become hyperthyroid again due to remaining thyroid tissue (although this is not common).

It is a safe, effective treatment that has been used in cats for many years. There are a number of referral centres offering this treatment with our closest in Chester. ‚Äč All in all with screening and treatment, it costs around £2500-3000.

Surgery (Thyroidectomy)

Removing the thyroid glands containing the tumours surgically is a very effective therapy and was considered 'Gold Standard' until radioactive iodine therapy became available. Greater than 80% of cats will have the disease in both their glands so most will have both glands removed.
It is delicate surgery and is performed by our more experienced surgeons. The parathyroid glands sit on top of the thyroid gland and help regulate calcium so cats are monitored for 48hrs after surgery with blood tests to ensure these glands are still functioning. It is not particularly painful surgery requiring a small incision over the neck.

A general anaesthetic is required and some of these patients are quite frail but we have excellent facilities, monitoring equipment and talented nurses. If we feel that a patient isn't a suitable candidate for surgery then we would consider another treatment option.

Like radioactive iodine, all cats will become transiently hypothyroid and some will require long term thyroid hormone supplementation later in life. Also there can be 'ectopic' thyroid tissue in the cat's chest that can result in the cat becoming hyperthyroid later in life.  Expected costs for diagnosis, surgery and follow up monitoring would be in region of £1500.

Medical Management

The medications we use to initially stabilise the cats initially can be used longer term. However, it is important to understand that this does not remove the tumour (unlike radioactive iodine or surgery). Side effects of the medication are not uncommon and can include the following:

  • Skin excoriation (scratching violently, normally around neck and head)
  • Blood cell issues - anaemia, immune system dysfunction
  • Liver toxicity

If your cat becomes unwell while on thyroid medication the safest thing to do is stop giving the medication and call us.

Thankfully these side effects usually wear off quickly when you stop the medication. It is for this reason that we need to check these cats frequently (including blood screens every 3 months). 
Medical management is good for cases that are not fit enough to tolerate radioactive isolation for 2 weeks or a general anaesthetic. For example, cats with advanced heart or kidney disease. The medication can be adjusted to the right level to stop the thyroid level dropping too low (hypothyroidism) which is better for cats with advanced kidney disease. 

Iodine Restricted Diet

Thyroid hormone contains Iodine so if you feed very little of this element to a hyperthyroid cat it will not be able to overproduce thyroid hormone. However the cat still has its tumour and if it eats any other food (including treats, mice, birds etc) it will have enough iodine to become hyperthyroid again. The diet also takes a few months to deplete the cat's iodine reserves enough to have an effect so they may require medication for stabilising on top of the diet initially. We tend to reserve this treatment option for indoor-only cats that live on their own that cannot undergo the other treatments above due to ill-health or side-effects. Costs for diagnosis, monitoring and food for the year is approximately £1000.

This information is just a summary of what is a very complex condition. If you would like more information please contact Steve at our Whitchurch branch who has a certificate in Internal Medicine and an keen interest in this disease.

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