Hyperthyroidism – or an overactive thyroid – is an extremely common disease of older cats, occurring in about 1 in 10 cats over the age of 10. Unlike many diseases, hyperthyroidism is more common in moggies than purebred cats. It is a progressive disease that will eventually damage organs, leading to high blood pressure, heart problems, and kidney problems amongst other things.

What is hyperthyroidism?

Hyperthyroidism is a condition caused by an overproduction of thyroid hormones, which are needed to regulate metabolism. This usually occurs because the thyroid gland (located in the neck) gets a benign tumour-  as the tumour grows, it throws out extra hormones. Unfortunately, an overproduction of thyroid hormone impacts the metabolism by causing an increase in metabolic rate. 

What are the symptoms of hyperthyroidism in cats?

An increased metabolic rate affects the whole body. You may notice that your cat is losing weight despite having a ravenous appetite. You may also notice them having more energy. Because an appetite and energy are usually seen as good things, these signs are often misinterpreted by owners as the cats having renewed youth. 

Other signs include an increased heart rate, dull fur with matts or dandruff, drinking more, and urinating more. You may also notice your cat making more noise – yowling is another common sign. 

What causes feline hyperthyroidism?

We don’t really know what causes hyperthyroidism. It was first discovered in 1970 and since then has become the most common hormonal condition of cats in the UK.  Despite many studies into cause, vets are still unsure what is causing the disease. It’s possible that goitrogens in food or water are causing the problem, but there’s currently no good evidence for this.

How is Hyperthyroidism in Cats Diagnosed?

Your vet will first do a physical exam, which will confirm the presence or absence of some of the common signs of hyperthyroidism. They’ll also need to check your pet over and see if anything else is happening. They may find a goitre – a lump in the neck where the tumour is growing – or notice changes in the eyes.

They will then need to do a blood test. This involves clipping a patch of fur from the neck or from a leg, then inserting a needle to draw 2mls of blood. Your cat will need to be taken out the back with a nurse for this, and some may need sedating. The blood will be tested for several diseases, as old cats can be prone to having several things go wrong at once. It also makes sure that medication is safe to give by checking other organs. Hyperthyroidism is diagnosed if the blood values for a substance called T4 are high. If T4 is normal, but your vet still suspects hyperthyroidism, they can send the blood to a lab for further tests.

What is the treatment for feline hyperthyroidism?

Hyperthyroidism has four major methods of treatment, some of which may not be suitable for some cats. 

Hyperthyroidism can, in some cases, be treated with a change in diet. A special diet with ultra-low iodine is used. Since T4 is made with iodine, if you restrict iodine you restrict the T4. This doesn’t work for cats living with other cats, or those who are fussy, or those who are fed treats; if they have any food that’s not the special diet, it won’t work.

Most cats with hyperthyroidism instead end up on daily medication. There are several types, given daily or twice daily, and they come in tablet and liquid form. The amount of medication your cat needs will be titrated to the T4 in their blood. So, your cat will need to return for regular blood tests to make sure the right amount is being given. This means that this route can become expensive over time, and may not be suitable for those cats that need sedating for every blood test.

The thyroid can actually be removed, also. This requires a general anaesthetic but can otherwise be done in many veterinary practices. It’s best suited for cats with a tumour in one side of the gland – removing both sides can be difficult and cause ongoing problems.

The newest available treatment is radioactive iodine therapy. This requires an extended hospital stay at a specialist centre and can be expensive. There are currently only a few hospitals offering this treatment in the UK. Your cat will be injected with radioactive iodine, which will be taken up by the thyroid gland. Once there, it damages the excess thyroid tissue and causes it to die off, leaving the healthy tissue behind. As with surgery, this is permanent. Although it is expensive, it should pay for itself after 3-4 years compared to daily medication. And because the result is permanent, you won’t have to continue going to the vet- great if you have a cat that’s scared.

You should discuss the treatments available with your vet – they will be able to help you make a decision that’s right for you and your cat.


Hyperthyroidism is a serious and life-threatening disease when it goes untreated. So, it’s a good idea to keep an eye on your older cat, watch for the symptoms listed above, and don’t forget to take them for their annual health checks.