Hyperthyroidism is a malfunction of the thyroid gland in which too much thyroid hormone is produced. The thyroid is a paired gland in the neck. Approximately 70% of cats with hyperthyroidism have both glands affected.

Hyperthyroidism is common in older cats, usually older than 8 years of age, and affects both sexes and all breeds. Hyperthyroidism is rare in dogs.

Benign (non aggressive) thyroid tumors, account for over 95% of clinical cases of hyperthyroidism. Increased thyroid hormone levels can affect many body organs.

Typical symptoms include;

  • Weight loss
  • Increased thirst
  • Increased urination
  • Increased heart rate
  • Hyperactivity
  • Increased hunger
  • A change in vocalization
  • Occasional diarrhoea or vomiting

Because the body must try to keep up with its metabolism, the heart will start to pump harder. This can cause increased blood pressure (hypertension) which can damage some of the organs of the body. In some cats the blood pressure can become so high as to damage the retina of the eye. This results in blindness.

The heart also becomes enlarged in order to meet its increased workload. This change can often be heard as a heart murmur. While treatment of the disease can reverse the changes to the heart, if left untreated your cat’s cardiovascular function will progressively worsen, and may be fatal.

Hyperthyroidism is usually diagnosed by a clinical examination with your vet and blood tests. The thyroid hormone levels will be measured and a general blood panel will be performed.


There are 3 treatment options available for hyperthyroidism, but not all are suitable for all animals.

Neomercazole (Carbimazole) tablets

Your pet would need tablets daily for the remainder of their life. The tablets inhibit the release of thyroid hormone. The dose may be adjusted after routine blood tests to check the levels of thyroid hormone. Neomercazole is a relatively inexpensive drug.

Radioactive Iodine Treatment

Your pet would receive an injection of a radioactive form of iodine. This would require a minimum of a 5 day stay in the veterinary clinic. Your pet will be referred to a different clinic for this treatment, as specialised facilities are required. Your cat has a 90% chance of responding favourably to a single injection. Other cats may require a further course of treatment. Cats that are exhibiting any evidence of kidney disease cannot be treated by this method, as fully correcting the hyperthyroidism may induce kidney failure. The cost of this treatment varies, please discuss with your Veterinarian.


This involves the surgical removal of the affected thyroid gland. However, this is only suitable if one thyroid gland is involved and the procedure has a potentially high complication rate. As radioactive iodine treatment is less invasive and often has good results, surgery is very rarely, if ever, performed now.

Many pets can maintain a good quality of life if their hyperthyroidism is treated or controlled. Your veterinarian will help you to decide which is the best treatment option for your pet.

Your pet’s heart may be affected with hyperthyroidism and there may be heart medication prescribed.

Your cat may also require regular blood tests to check the levels of the thyroid hormone.