Hyperthyroidism in Cats

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Hyperthyroidism is a relatively common problem in older cats. It is caused by an overactive thyroid gland, which produces excessive amounts of thyroid hormone. This hormone has an effect on many organs in the body causing a range of clinical signs.

Clinical Signs

Clinical signs of hyperthyroidism can be variable and not all cats are affected in the same way. Common signs include

      • weight loss with normal to increased appetite,
      • increased activity level,
      • vomiting,
      • change in defaecation pattern, and
      • poor hair coat.

There are also a small number of affected cats, which actually have a reduced appetite and activity level and do not lose weight.

The thyroid hormone also has an effect on the heart, causing a heart muscle disease that can lead to heart failure.

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Hyperthyroidism is diagnosed easily by a blood test to check the level of thyroid hormone in the blood. It is important to also check for other illness as this can sometimes interfere with the diagnosis, as well as providing important information about the general health of your cat.


Hyperthyroidism is a treatable disease. Most commonly it is treated with tablets for the rest of your cats life. Some cats however do not tolerate the tablets, so for these cats, treatment can be given as a transdermal gel. A small amount of medicated gel can be placed on the cat's ear and is absorbed through the skin. There is also a once off treatment with radioactive iodine available once cats have been initially stabilized and are doing well.

Ongoing Monitoring

4414905 sOnce treatment is started, it is important to monitor thyroid hormone levels on a regular basis to ensure the dose is correct for your cat. As 95% cats are over 10 years of age when diagnosed with hyperthyroidism it is also important to monitor other parameters in the blood when they are on treatment. Once the thyroid disease is being controlled, other diseases especially kidney disease can be uncovered which the thyroid disease may have been masking. If this is the case then the two diseases need to be managed together.


Once they have been stabilised, most cats go on to lead and normal healthy life with no ongoing problems related to the thyroid.

If you think your cat may have thyroid or any other problems, please phone the clinic for a appointment.