• Dr Cori

Hyperthyroidism in Cats.

Hyperthyroidism is a very common condition seen in cats of both sexes from around 10-12 years old.


The clinical signs are caused by an excess of thyroid hormone in the body, which is caused by a growth on the thyroid gland, most often a benign growth called an adenoma.

This extra thyroid hormone causes signs such as weight loss (often marked) increased activity, increased thirst and urination, increased hunger and a dull or greasy coat. Around half of the cats who have hyperthyroidism will also present with vomiting.


These signs are often very slight to start with but become more pronounced as the disease progresses.

Thankfully it is fairly easy to diagnose. Your veterinarian will do a clinical examination and blood tests to check thyroid levels and often a baseline panel to assess kidney and liver function among other things.


Sometime a test will come back as normal, if your vet is still suspicious your cat has hyperthyroidism there are further confirmatory tests that can be performed.


If the tests come back positive then we need to treat! There are a few different treatment options available:



Diet treatment:

Cats are placed on an iodine reduced diet for the rest of their life. Iodine is needed to make thyroid hormone, so if there is less iodine, then there will be less thyroid hormone production. This only works in some cases and is best suited to indoor only cats as they need to eat only the diet and no extras for it to be effective. If the thyroid levels do not return to normal on diet alone, medical therapy is indicated.


Medical therapy:

This reduces the production of thyroid hormone in the thyroid gland. Two options are available, tablets or a topical gel.

The tablet option is given once or sometimes twice daily. The pill is small and easy to hide in food.

The other option is a topical transdermal gel which is placed on the ear and is a very effective treatment, particularly in cats who hate tablets!

Either of the medication options can be used long term to control the signs. It is not a permanent fix and medication must be given daily for the rest of the cat’s life or the signs will return.

We recommend a blood test every 6 months to check the thyroid levels and kidneys are functioning normally. It can take a little while with medication doses to find the perfect dose for your cat, and your veterinarian will alter the doses based on the most recent blood tests.

Sometimes as the disease progresses, doses may need to be increased in previously well controlled cats, so if you notice a change in behavior or any of the clinical signs returning be sure to go back to your vet for a recheck.


Radioactive Iodine therapy:

This is a permanent solution to the hyperthyroidism. Cats are admitted to a vet hospital for a minimum of one to two weeks. A radioactive iodine injection is placed under the skin that gets absorbed into the bloodstream. When the thyroid takes up the iodine (needed in the production of thyroid hormone) the radiation destroys the thyroid tissues without affecting any other tissues around it.

Most (around 95%) cats have normal thyroid levels after this procedure. However if unsuccessful, the treatment can be repeated. On rare occasions the treatment can be a bit too successful and we see the opposite condition of hypothyroid (thyroid levels too low) but this can be treated with oral thyroid supplements.

Due to the dangers of radioactive material to people, you will be unable to visit your cat during the week(s) spent at the vet clinic for treatment.


The overall outlook (prognosis) for cats with hyperthyroidism is good if the condition is treated, controlled and your cat is well monitored with blood tests and health checks.

If untreated sadly hyperthyroidism can lead to further problems such as increased blood pressure and heart disease. This is a snowball effect as the high blood pressure can affect kidney and retina function.

As you can see, the consequences of not treating a hyperthyroid cat is severe and their quality of life would be majorly affected with these side effects.


If you think your cat is showing any of the signs of hyperthyroidism make an appointment with your regular veterinarian.



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