Hyperthyroidism in Cats

Hyperthyroidism is a common condition, often affecting older cats.

The thyroid gland is a small gland that sits within the neck; it is responsible for producing thyroid hormones which control our metabolism. With hyperthyroidism there is excessive production of these hormones which result in an increased metabolism. This means the body is burning through more energy.

This presents as weight loss in affected cats, despite having a ravenous appetite. Affected cats will often be quite restless, and possibly irritable and vocal. Affected cats may have increased thirst with subsequent increased urination also. Some cats may also develop vomiting or diarrhoea.

If left untreated hyperthyroidism will result in high blood pressure (known as hypertension) which can have detrimental effects on the body. It can also result in changes to the heart in some cases.


Hyperthyroidism is usually easily diagnosed by a blood test in which we measure the level of thyroid hormone in the blood. If this is increased above normal then the cat is diagnosed with hyperthyroidism.

There are secondary blood tests we can also perform if necessary to help achieve a diagnosis.


Because this is such a common disease of cats there are a variety of treatment options available.

  1. Surgical removal of the affected gland.
  2. Dietary therapy – the Hills y/d diet aims to control hyperthyroidism by having a reduced iodine content which stops the body from producing excessive thyroid hormone. For this diet to work the cat can only eat this food and none other. If the cat eats any other food, even a small amount, it will render the y/d diet ineffective. In many cases the diet won’t achieve as good control as other options.
  3. Medical treatment – this involves giving medication which prevents production of the excessive levels of thyroid hormone to reduce the metabolism back to normal. Medication needs to be given daily and can either be given as an oral tablet, or a gel that is rubbed onto the skin on the ear and absorbs into the blood stream through the skin.
  4. Radioactive iodine therapy. This involves giving an injection of radioactive iodine which is taken up by the abnormal part of the thyroid gland and destroys it. It does not affect any other part of the body. This cures the cat of the disease in 95% of cases. In 5% of cases it may recur. We have to refer cats to VSG for this process as we cannot perform it here, and there is often a short waiting list, but it is a very effective procedure. We have had many cats who have been through this process and are now subsequently cured of their hyperthyroidism and back to normal health.

So if you have noticed weight loss in your cat despite a seemingly normal or increased appetite, as well as restless behaviour, vomiting, or diarrhoea, then arrange an appointment with a vet and we can investigate.

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Kohimarama Vets

325 Kohimarama Road
St Heliers, Auckland 1071