Photo of a Russian Blue cat running in the grass.

Feline Hyperthyroidism

The thyroid is an organ in the body that is responsible for production of two hormones: thyroxine (T4) and triiodothyronine (T3), which help to regulate the metabolism. T4 is the most common hormone, but T3 is the actual functional hormone. Therefore, any T4 that the thyroid gland produces has to be converted to T3 by the liver before it can be active.

The body needs iodine to make thyroid hormones. It controls the rate of hormonal production based on need and current illnesses, however, the thyroid gland itself can become diseased and make too few hormones (hypothyroidism) or too many (hyperthyroidism).

Cats rarely suffer from hypothyroidism, but hyperthyroidism is quite common, and affects cats of all ages, despite being seen more often in aging cats.

Symptoms of Hyperthyroidism in Cats

Clinical signs of hyperthyroidism vary based on the individual cat, but the most common are:

Any cat showing clinical signs or suspected of suffering from hyperthyroidism should be evaluated by a veterinarian to further diagnose the condition. On a physical exam, a veterinarian may be able to feel enlarged thyroid glands in the neck region of affected cats. In addition, they’ll look for evidence of other diseases with similar clinical signs.

Hyperthyroidism is diagnosed by sending out bloodwork to look at T4 levels, since cats with hyperthyroidism will have an elevated T4. In a smaller subset of cats, T4 levels will be normal, despite showing clinical signs that match hyperthyroidism. In this case, additional hormonal testing will be recommended.

Baseline blood work to look at liver, kidney, red blood cell count, and white blood cell count is commonly completed at the same time. It is important to evaluate renal (kidney) function in cats before and throughout the treatment process for hyperthyroidism. Increased circulation in the thyroid hormone can result in a larger amount of blood flow to the kidneys. This may mask underlying kidney disease and, when the metabolic rate gets closer to normal, it can reveal elevation of kidney values. Cats with underlying renal disease are often in the same age range as those with hyperthyroidism.

Treatment of Feline Hyperthyroidism

There are many treatment options for hyperthyroidism. Each one has pros and cons and should be discussed with your veterinarian based on your cat’s history.

Long-term complications of untreated hyperthyroidism can be secondary to heart disease. Cats with unregulated hyperthyroidism can also have an increase in heart rate and secondary thickening of heart muscles, which leads to development of a condition called hypertrophic cardiomyopathy. Hypertension (high blood pressure) can also develop as a secondary complication to untreated or uncontrolled hyperthyroidism.

With appropriate management of hyperthyroidism and prevention of secondary complications, cats with this disease can live long, healthy lives.

If hyperthyroidism is suspected in your cat, discuss testing and treatment with your primary veterinarian.