Hyperthyroidism in Cats

Our feline friends are living longer than ever, and as they age, we see the emergence of certain conditions, including an over active thyroid gland. Hyperthyroidism stems from the thyroid gland over-producing thyroid hormone, leading to a host of problems, ranging from weight loss, heart disease, kidney disease and other issues. The good news is that this common problem can be managed and even cured in many cats.

A cat’s body needs a normal amount of thyroid hormone: it regulates metabolism, heart rate, body temperature, overall body condition, and more. It is important for normal functioning, but too much of a good thing can be bad. Excessive thyroid hormone leads to increased metabolism – the body burns too many calories, resulting in weight loss. The elevated metabolism also increases blood pressure, leading to possible long-term kidney and heart damage. Problems may also be noted in the eyes and brain.

The thyroid is a small organ, located in the neck on top of the trachea (windpipe). Usually it cannot be felt, but in some cases, especially when there is hyperthyroidism, it can palpate the size of a pea. Each cat has two thyroid glands.

Clinical signs associated with an overactive thyroid may vary between cats, but may include weight loss, increased appetite without weight gain, vomiting, diarrhea, hyperactivity, increased heart rate, behavior changes, poor haircoat, increased drinking and urination, occasionally depression or weakness, and difficulty breathing. Many of these observations are also present in other disorders, so one cannot diagnose thyroid disease purely upon clinical signs.

Diagnosis of thyroid disease is performed by your veterinarian through a physical examination, thorough history, and evaluation of blood values, specifically thyroid hormone. Most cats with the disease will have very high levels of thyroid hormone, but some may be marginally elevated or high normal. Thyroid hormone levels can fluctuate some, so if it comes back normal, your veterinarian may want to check the level again shortly after the initial examination.

The cause of hyperthyroidism is not definitively known, but the incidence of the disease has increased over the past decades. Researchers are looking into whether a fish-based diet may increase the incidence if the disease. PBDEs, a chemical used in fire retardants is commonly found in dust mites and sometimes in cat foods with a high fish content. There is thought that this chemical may contribute to hyperthyroidism.

There are several methods for treating hyperthyroidism. The goal is to bring down the level of thyroid hormone to a normal value. A medication called methimazole reduces the amount of hormone released by the thyroid gland. It is usually used first, with the veterinarian carefully monitoring the thyroid level, kidney values and blood cell counts several times to be certain an appropriate dose of the medication is administered. Many cats are kept on this medication for prolonged periods of time.

If the cat cannot tolerate methimazole, if giving the medication is difficult, or if a long-term solution is the goal, an injection of radioactive iodine may cure the disease. Iodine is taken up by the thyroid gland from the bloodstream and is used to make thyroid hormone. The iodine is very dilute in the body, but concentrates in the thyroid, essentially irradiating it, reducing the amount of thyroid tissue present. Many cats will quickly have a normal thyroid level or slightly low level. If the treatment is not effective, it may be repeated. The downside is that cats must remain in the hospital for 3-4 days while all the radioactivity dissipates and is urinated out.

A new veterinary diet called y/d is available for the treatment of hyperthyroidism. It contains only trace amounts of iodine, a mineral essential to the formation of thyroid hormone. Without sufficient quantities of iodine, the thyroid can only produce small amounts of thyroid hormone. This diet may be good for cats that are difficult to medicate or have other medical problems. It is essential that the cat have filtered water and eat no other foods, not even treats, as even trace amounts of iodine can negate the purpose of the food.

In some cases surgery is performed to remove the affected thyroid gland. The surgery must be performed carefully to preserve the parathyroid – a small gland adjacent to the thyroid that regulates blood calcium. There can be small areas of thyroid tissue not connected to the thyroid and unless these areas are identified in advance, surgery may not be successful.

Hyperthyroidism is a treatable disease. If there are no other significant complications, cats may go on to live long lives with treatment and supportive care as needed. If your middle-aged to older cat is losing weight, thyroid disease should certainly be a differential.