Feline Hyperthyroidism

The thyroid is an organ in the body responsible for producing two hormones: thyroxine (T4) and triiodothyronine (T3), which help regulate the metabolism. T4 is the most common hormone, but T3 is the actual functional hormone. Therefore, any T4 the thyroid gland produces must 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:

• Weight loss, despite ravenous appetite. If there is an excess of thyroid hormones, the metabolism will be increased, leading to greater appetite coupled with unexpected weight loss.
• Vomiting
• Hyperactivity
• Increased vocalization
• Unkempt, poor hair coat
• Increased urination/thirst

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 more 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.

• Daily medication. Anti-thyroid medications help to decrease production and release of thyroid hormones. These medications do not cure hyperthyroidism but reduce the amount of circulating thyroid hormone. They need to be given daily (once or twice), and a strict schedule has to be maintained. Skipping doses causes the thyroid hormones to rise. There are transdermal options that may be used, but these also need to be applied daily. These medications tend to be inexpensive, and long-term monitoring with blood work is required to monitor T4 levels and adjust medications based on the patient’s needs.
• Radioactive iodine therapy. This treatment involves giving an injection of radioactive iodine into the thyroid glands, which destroys the overactive tissue. It must be done at a specialized facility and the patient needs to stay at the facility until their radioactive levels have dropped. Treatment is highly successful, with 95% of cats being cured, and many cases do not require daily medication afterwards. In a small number of cases (<5%), an additional dose of radioactive iodine is needed months after the initial treatment. Some cats will develop hypothyroidism (low thyroid hormone) and need thyroid supplementation afterwards, but this is only seen in a small percentage of cats.
• Surgical removal of thyroid glands. The goal of this surgery is to remove the thyroid glands, and it needs to be done by a skilled surgeon and accompanied by general anesthesia. This is usually curative, but surgery runs the risk of accidental removal of the parathyroid glands, which can create a different hormonal imbalance. Surgical removal of the thyroid glands is not a commonly recommended treatment for feline hyperthyroidism.
• Dietary management. A new iodine-restricted diet released by Hill’s Science Diet (called Y/D) acts to prevent iodine intake, so that the thyroid glands cannot produce T4 or T3 hormones. This diet can be a successful treatment of hyperthyroidism, but compliance is key. It needs to be the only food consumed by the pet for the remainder of their life. This can be difficult for multicat households or picky eaters.

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.

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