Hyperthyroidism in Cats

Hyperthyroidism is the most common hormonal illness affecting cats in Australia. It usually affects older cats and is caused by an excessive production of thyroid hormone by the thyroid glands in the neck.

What causes hyperthyroidism?

Hyperthyroidism in cats is caused by an overactive thyroid gland. Few cases of hyperthyroidism in cats are caused by cancerous thyroid tissue, most result from hyperplasia (excess growth) of the thyroid gland or benign functional tumours causing excess production of thyroid hormones. In most cases the glands on both sides are effected. The excessive thyroid hormones lead to an increase in overall body metabolism, affecting many organs of the body.

Hyperthyroidism is primarily a disease of cats. It rarely is diagnosed in dogs. Usually middle-aged to older cats are affected, and both male and female cats may develop hyperthyroidism.

What signs do you see?

• Excessive thirst and appetite
• Weight loss
• Hyperactivity or restlessness
• Change in temperament (a normally docile cat may become aggressive)
• Excessive urination, or urination in the house
• Vomiting
• Poor hair coat

How is hyperthyroidism diagnosed?

Hyperthyroidism is diagnosed by a blood test, which can be performed by your veterinarian, although the result may need to be checked with a further test, sometimes performed by a medical specialist. If you suspect your cat is showing any of the symptoms, it is highly recommended to consult your vet as further problems can occur, including fatal heart complications.

Is there treatment for hyperthyroidism?

Treatment options include drug therapy, surgery, and radiotherapy.

Radioactive iodine therapy has a high success rate, and involves admitting your cat to a specialist hospital where it will be given a radioactive iodine tablet. Since your cat will be radioactive for 10 days, it needs to stay in hospital for that long, and you cannot visit it and it will only receive minimal attention from staff.

Drug therapy controls the condition rather than curing it. You will need to give a tablet 2 or 3 times a day, but it is an option for owners who consider that their cat is too old and frail to survive the hospital stay for the other treatments, although there are some potential side effects to this form of treatment.

Surgery involves removal of the thyroid gland but this is risky surgery which needs to be performed by an experienced specialist surgeon, and is not commonly performed.

If treatment is successful, by whichever method, it is often amazing the difference it makes. Cats quickly become more relaxed and healthy, and back to the business of appreciating life. If left untreated, the increased metabolic rate, weight loss and weakness will become more severe and the cat will eventually succumb to heart failure.