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Hyperthyroid in Focus on Cats

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Hyperthyroidism in Focus


Hyperthyroidism is an extremely common disease of older cats, occurring in about 1 in 10 cats over the age of 10, and unusually for most diseases appears to be more common in moggies than purebreds. It is a progressive disease that does a lot of damage to various organs, so it’s an important one to keep an eye out for. We recommend health checks at least annually in all our patients, although in older cats every 6 months would be ideal.

What is hyperthyroidism?

Hyperthyroidism is an overproduction of thyroid hormone from the thyroid gland in the neck, usually due to a benign tumour of the gland. As thyroid hormones regulate metabolism, an overproduction of the hormone causes an increase in metabolic rate. This causes things like an increased heart rate, increased appetite, and increased activity levels, although many cats won’t show many symptoms. The disease is progressive and will get worse without treatment- it can quickly cause heart problems due to the increased strain on the heart.

We don’t really know what causes hyperthyroidism. It appears to be on the increase to an extent (it was first found in 1970 and since then it has increased to be the most common hormonal disease of cats) but how common it is varies from country to country. Several studies have been done to look into the disease but so far there have been fairly inconsistent results. Most of the studies have looked closely at nutritional factors and known goitre-causing toxins in food and drinking water, as these have been linked to a similar disease in people.

What symptoms should I look out for?

It is rare to diagnose hyperthyroidism in cats under the age of 10, but once cats get over ten the chances are quite high that your pet will develop hyperthyroidism. For this reason, we recommend keeping an eye out for common signs at home. The symptoms of hyperthyroidism include an increased appetite along with weight loss, manic behaviour including increased vocalisation, and increased drinking. More subtle signs include a poor hair coat, increased urination, and a fast heart rate. You may also notice signs of heart disease such as panting or inability to exercise. If you notice any of these signs we recommend checking your cat over as soon as possible.

What will you do?

The first thing we will do is ask you about the symptoms you’ve noticed at home. Once we’ve done that we will check your cat over thoroughly. Hyperthyroidism usually causes changes that can be picked up during a clinical examination (such as an increased heart rate, weight loss, and a ‘goitre’, or lump in the location of the thyroid gland). However, it’s possible that other diseases cause the same symptoms. If we suspect hyperthyroidism we will suggest a blood test to confirm our suspicion. This is a quick procedure (as long as your cat is amenable!) and you will only need to leave your cat with us for a few minutes. The blood sample will tell us about levels of thyroid hormone- if they’re high, it’s likely your cat has hyperthyroidism. We also recommend checking for other diseases- it is common for older cats to have kidney problems so it’s worth checking those- and also checking that they can undergo treatment safely.

So it can be treated?

Definitely! And the great news is that treated cats usually reverse all symptoms. There are four major avenues to explore for treatment- all are valid and may or may not work in each individual circumstance.

Here at Churchcroft vets we often perform surgical removal of the thyroid gland. The tumorous gland is removed and without it the thyroid hormone in the blood reverts to normal. We will want to check the levels and do some other blood tests after surgery, however the good news is that this solution is usually permanent. Some cats will have tumours in both glands. Both glands can be removed if necessary, but this is slightly more complex and the chances of side effects are much higher- we will discuss these with you if we think your cat might have the disease in both glands. There are also some cats that cannot undergo surgery for one reason or another, and for these cats the other options for treatment may be more valid.

One such treatment is radioactive iodine therapy. This requires an extended hospital stay at a specialist centre and as such can be expensive. Your cat will be injected with radioactive iodine, which will only be taken up by thyroid tissue. This then 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 is suggested that it pays for itself over 3-4 years of daily medication, and like surgery has the advantage of fewer vet trips due to being a permanent solution.

Another method of treatment is daily medication, and sometimes this method is combined with the other methods temporarily. How effective daily medication is depends entirely on your cat. Most medications are tablets- either daily or every 12 hours- but there are also liquid formulations available. If your cat won’t take medication or disappears for days at a time, this option is probably not for you! Medication is cheaper in the short term than either option discussed so far but does require repeat visits to see us. We will need to check your cat’s thyroid hormone level regularly to ensure the dose is correct, and regular checks to ensure the medication isn’t damaging any other organs is important too. If your cat won’t take an oral medication, there is the option for a topical cream. This needs to be applied to the ear, and is usually tolerated well. However, levels of absorption can change day to day, which means that control with this method is slightly harder than with tablets. The regular checks will still be needed and may even be more frequent as accurate dosing is difficult.

The final option is to try to control the goitre with diet. Since thyroid hormones are made from iodine, restricting the iodine can stop excess thyroid hormone from being made. This really only works in indoor-only cats as if the cat gets iodine from any other sources- next door, hunting, the odd treat or bit of human food- then they will go back to being hyperthyroid. Many cats are fussy and won’t eat the food, so this option may not work for them. It’s not a commonly used solution as the other methods are more likely to work, but it can be effective for some cats.


Whichever option you choose (and if one isn’t working, feel free to discuss it with us and opt for another) it’s important to make sure your cat gets treated. We’ll support you in your decision making- please just ask us if you aren’t sure or if you need more information. Hyperthyroidism is a serious and life-threatening disease when it goes untreated and is far too common to go ignored. So keep an eye on your older cat, watch for the symptoms listed above, and don’t forget to bring them in for their health check.

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