What is hyperthyroidism?

Hyperthyroidism is a common disease affecting older cats in which the level of thyroid hormone (T4) in the body is too high. It is usually caused by a benign tumour of the thyroid gland.

What are the signs of hyperthyroidism?

A cat with hyperthyroidism may show some or all of the following signs:

  • Weight loss
  • Increased appetite
  • Poor coat quality
  • Hyperactivity
  • Vomiting or diarrhoea

How is hyperthyroidism diagnosed?

If we suspect hyperthyroidism, we will run a full blood and urine panel. This will confirm the diagnosis of hyperthyroidism and also alert us to any concurent problems such as renal disease that may alter the recommended treatment.

How is hyperthyroidism treated?

We will usually start by prescribing an 'anti-thyroid' drug that, given daily, will block the production of thyroid hormone and bring the hormone levels back towards normal. This medication can be given as a tablet or a gel that can be applied to the inner surface of the ear. It does not cure the disease, but controls clinical signs. We then monitor the T4 level monthly and alter the dose accordingly until we reach an appropriate dose to keep the T4 level within the normal range. We also monitor kidney function during this time, because hyperthyroidism can sometimes mask underlying kidney disease.

If there is no underlying kidney disease and daily treatment is a hassle, we can consider a more permanent treatment to cure hyperthyroidism. This involves referral to a specialist clinic, where your cat will be given a dose of radioactive iodine. The iodine concentrates in the thyroid gland and selectively destroys thyroid tissue. For radiation safety reasons, your cat will then require boarding for 7 days. The risk of this procedure is that we may induce a hypothyroid state (where the thyroid gland produces too little thyroid hormone). Surgical removal of the thyroid gland has been used as a treatment for hyperthyroidism, but can be associated with dangerous complications.