A deficiency of thyroid hormones is called hypothyroidism. While this condition is much more common in dogs, hypothyroidism can sometimes occur in cats and is usually an adverse effect, called iatrogenic hypothyroidism, that has developed as the result of treatments for overproduction of thyroid hormones, which is referred to as hyperthyroidism. If your cat has become lethargic and gained weight, especially after being treated for hyperthyroidism with iodine 131 therapy, antithyroid drugs, or thyroidectomy, it is important for you to contact your veterinarian immediately as hypothyroidism can result in many serious health problems, including but not limited to, kidney disease.

As in other animals, the thyroid gland in a cat is located on both sides of the trachea or “windpipe.” The thyroid is controlled by the pituitary gland, which sits near the base of the brain. The thyroid’s purpose is to release hormones such as thyroxine (T4) and, in lower amounts, triiodothyronine (T3) that control the body’s metabolic functions. Metabolism is the process through which the body converts nutrients into energy and then flushes out the waste. Deficiency in these metabolic hormones can result from a thyroid gland that is not producing enough hormones, a thyroid that cannot correctly synthesize or secrete the hormones, or a thyroid that did not adequately develop.

Dependent upon whether the hypothyroidism is thought to be iatrogenic or congenital, the symptoms may appear differently.

Thyroid hormone deficiency in cats is quite rare. When it does occur, it is almost always the result of iatrogenic hypothyroidism, meaning that it is an adverse reaction to treatment for the thyroid’s overproduction of hormones. There are also some even rarer cases hypothyroidism that are congenital in nature.

Iatrogenic hypothyroidism is caused by the lowering of thyroid hormone production that can occur after:

Congenital hypothyroidism, which is extraordinarily rare in cats, is generally caused by one of two inherited genetic disorders:

Your veterinarian will begin by asking you what symptoms you have noticed in your cat and by reviewing your cat’s history. If your cat has recently been treated for hyperthyroidism, the vet will likely begin tests to determine if your cat has developed iatrogenic hypothyroidism. These tests include:

Some cases of iatrogenic hypothyroidism may resolve without medical treatment after treatment for hyperthyroidism has ended, while some cats with iatrogenic hypothyroidism may need long term treatment, including synthetic hormone medication. If secondary illnesses, such as kidney disease, have developed as a result of the hypothyroidism, treatment will be necessary for those conditions as well. It is not uncommon for the secondary conditions to dissipate once hormonal balance has been achieved. Congenital hypothyroidism will likely require medical treatment for the duration of your cat’s life. Treatments for one or both forms of thyroid hormone deficiency in cats may include:

After one or two months of hormone replacement therapy, a cat that is responding well to treatment will likely begin to have a healthier coat, be more mentally alert, and return to a healthy body weight. Most cats, if they are otherwise healthy, will make dramatic improvements as a result of hormone replacement therapy. If you do not observe these improvements in your cat’s symptoms after the treatment has had adequate time to take effect, contact your veterinarian. Even if your cat makes marked improvements in their health, you will need to remain consistent in administering medication and with follow-up appointments with your vet.