What is hyperthyroidism?

Hyperthyroidism is the most common glandular disorder in cats. It is caused by an excessive concentration of circulating thyroxine—a thyroid hormone better known as T4—in the bloodstream.

What are the symptoms of hyperthyroidism?

Weight loss and increased appetite are among the most common clinical signs of this condition. Weight loss is seen in 95 to 98 percent of hyperthyroid cats, and a hearty appetite in 67 to 81 percent. Excessive thirst, increased urination, hyperactivity, unkempt appearance, panting, diarrhea and increased shedding have also been reported. Vomiting is seen in about 50 percent of affected cats. Clinical signs are a result of the effect of increased T4 levels on various organ systems.

What breeds/ages are prone to hyperthyroidism?

Hyperthyroidism can occur in any breed of cat, male or female, but occurs almost exclusively in middle-aged to older animals. Less than 6 percent of cases are younger than 10 years of age; the average age at onset is between 12 and 13 years.

How is hyperthyroidism diagnosed?

Because several common diseases of older cats—diabetes, inflammatory bowel disease, intestinal cancer and chronic kidney failure—share some of the clinical signs of hyperthyroidism, a battery of tests is in order. A CBC, chemistry panel and urinalysis alone will not diagnose hyperthyroidism, but they can certainly rule out diabetes and kidney failure. Hyperthyroid cats may have normal findings on the CBC and urinalysis, but the chemistry panel often shows elevation of several liver enzymes.

In the vast majority of cases, a definitive diagnosis of hyperthyroidism is based on a simple blood test that shows elevated T4 levels in the bloodstream. Unfortunately, between 2 percent and 10 percent of cats with hyperthyroidism will have normal T4 levels. One possible explanation for this is that in mild cases, T4 levels can fluctuate in and out of the normal range. Another is that concurrent illness will suppress elevated T4 levels, lowering them into the normal or high-normal range and fooling the veterinarian into thinking that the cat’s thyroid status is normal. Because these are geriatric cats, concurrent illness is fairly common, and diagnosis of hyperthyroidism in these cats can be tricky.

How is hyperthyroidism treated?

Several treatment options for hyperthyroidism exist, each with advantages and disadvantages.

– Oral administration of antithyroid medication: Methimazole (brand name TapazoleTM) has long been the mainstay of drug therapy for feline hyperthyroidism. It is highly effective in correcting the condition, often within two to three weeks; the dose is adjusted based on results of follow-up blood tests. Unfortunately, about 15 percent to 20 percent of cats will suffer side effects, such as loss of appetite, vomiting, lethargy, blood clotting problems, jaundice, itching around the head and face and, occasionally, blood cell abnormalities. Most side effects are mild and eventually resolve, although some necessitate discontinuation of the medication. Lifelong daily medication is required, which is a disadvantage to owners whose cats resist pilling. CBC, chemistry and T4 levels need to be rechecked regularly for the remainder of the cat’s life.

-Radioactive iodine therapy: This is the best and most sophisticated treatment option. Radioactive iodine, given by injection (usually under the skin), becomes concentrated in the thyroid gland, where it irradiates and destroys the hyperfunctioning tissue. No anesthesia or surgery is required, and only one treatment is usually needed to achieve a cure. Many treatment facilities are found throughout the country, and your veterinarian can refer you to the closest one in your area. Hospitalization may be prolonged; depending on local or state ordinances, cats may need to be kept at the treatment facility for 10 to 14 days until the level of radioactivity in their urine and feces decreases to an acceptable level.  Because some cats suffer decreased kidney function when thyroid hormone levels are decreased by treatment, it is advisable for all cats to be treated first with methimazole (see above) to test their kidneys’ ability to function with lowered hormone levels.

-Surgical removal of the thyroid gland: Hyperthyroidism is usually caused by a benign tumor called a thyroid adenoma that involves one or both thyroid glands. Surgery to remove an enlarged gland usually lowers thyroid hormone levels, but is not always curative as both thyroid glands may be overactive even if only one appears visibly enlarged. Surgery may be a good option for cats who cannot tolerate methimazole treatment, and who for medical or financial reason are not able to receive radioactive iodine therapy.

This information was adapted from an article written by Arnold Plotnick, DVM.


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