Hyperthyroidism in Cats

By Dr. Michael Averill, Veterinarian and Practice owner


Hyperthyroidism is a relatively common hormonal disorder in cats. Most cases (up to 99%) result from a benign (non-malignant) nodular enlargement of the thyroid gland(s) located in the neck. In approximately 70 to 75% of cats with hyperthyroidism, both thyroid glands are affected.

The incidence of this disease has increased steadily since the early 1980’s but the reason for this is still unknown. Multiple factors such as dietary, environmental, genetic and immunological causes may be involved.

Hyperthyroidism occurs most commonly in middle-aged to older cats, with an average age of onset of 13 years. The thyroid gland produces thyroid hormones which regulate the body’s rate of metabolism and in hyperthyroid cats there is an excessive production of thyroid hormones which causes an increased metabolic rate.

The most common signs seen in hyperthyroid cats are weight loss, increased appetite, vomiting, increased thirst and urination, restlessness, unkempt hair coat and diarrhea. Some cats may present with loss of appetite and weakness. Cats with hyperthyroidism commonly develop rapid heart rates, heart murmurs and high blood pressure. Left untreated these can lead to heart failure, kidney failure, liver failure or blindness.









In normal healthy cats the thyroid glands cannot be palpated in the neck but in most cats with hyperthyroidism an enlarged thyroid gland(s) can be felt. The diagnosis can be confirmed in most cats by a blood test to measure thyroxine (T4) hormone levels, which are usually high in cats with hyperthyroidism.

The good news for cats with hyperthyroidism is that only 1 to 3 % of cases are caused by a malignant cancer of the thyroid gland. Because most cases are benign nodules (adenomas), treatment of hyperthyroidism in cats is usually very successful. There are 4 choices of treatment available, which can be tailored to suit each individual cat:

Radioactive iodine - Radioactive iodine therapy is a simple, effective and safe treatment of choice in otherwise healthy cats with hyperthyroidism. We have a special license to treat cats with this by injection at The Straven Road Veterinary Centre. It destroys the abnormal thyroid tissue but does not damage the normal thyroid tissue. Cats require hospitalization after treatment, usually for 5 to 7 days. Dr Mike Averill has treated over 166 cats this way over the last 10 years. The oldest cat he has treated was 19 years old and another cat treated at 17 years of age lived until it was 21.

Surgery - This involves surgery to remove the abnormal thyroid gland (thyroidectomy). It is generally only safe to remove one thyroid gland in affected cats, because of the risk of damage to the adjacent parathyroid glands during surgery. The risk of anaesthesia in an elderly cat also needs to be considered.

Medical treatment - An orally administered anti-thyroid medication can be used to treat hyperthyroid cats. It can also be applied as a transdermal gel to the ear. It works by blocking the production of thyroid hormones, so it does not cure the disease and has to be given daily for the remainder of the cat’s life. Some cats can develop side effects such as lethargy and vomiting on this drug.

Change of diet - In recent years, a prescription diet has been developed to help manage hyperthyroid disease in cats by providing limited availability of iodine, reducing thyroid hormone production.  In many cats this is very effective as the sole treatment for hyperthyroidism. 

In conclusion, if your elderly cat is losing weight, remember to get it checked by your veterinarian, as it may be due to hyperthyroidism, not just old age. With appropriate treatment these cats will regain weight and can lead a normal, healthy life.

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