Feline Hyperthyroidism

Feline Hyperthyroidism

Feline Hyperthyroidism

By: Fielding D. O'Niell, DVM, MS
May 2012

By far, the most commonly diagnosed glandular disorder in cats is hyperthyroidism. The vast majority result from a benign thyroid adenoma that overproduces the hormone thyroxine. Only 1 to 2 percent are caused by a malignant thyroid carcinoma, however, recent published data indicate that up to 25% of benign adenomas may undergo malignant transformation.

The disease occurs at around 12 years of age and the classic symptoms are weight loss with a ravenous appetite. Other signs include vomiting, diarrhea, extreme thirst and hyperactivity. Secondary cardiovascular affects can be life-threatening. Ten percent of hyperthyroid cats, however, are the "apathetic" form. These cats exhibit lethargy, decreased appetite and weight gain; the exact opposite of the typical case.

This disease is easily diagnosed with a simple blood test and fortunately we have several treatment options:

Surgical removal of the thyroid gland was the first treatment developed. It is potentially curative but these cats are extreme anesthetic risks and the post-op complications can be life-threatening.

Radioactive Iodine Therapy is the treatment of choice in humans. It is potentially curative in the majority of cats; however, like surgery it is irreversible. Many hyperthyroid cats have hidden underlying kidney problems that are actually being helped by the excess thyroid hormone. A sudden irreversible drop in thyroid hormone levels may allow acute decompensation and kidney failure.

​​​​​​​Oral Methimazole Therapy blocks the formation of active thyroid hormone. The drug is inexpensive and side effects are rare. Also, unlike surgery or radioactive iodine treatment, the dose can be adjusted to suit the individual patient. Therefore, oral methimazole is my personal treatment of choice.

More recently, a diet deficient in iodine has been developed to "starve out" the thyroid gland. None of us have long term experience with this treatment but it appears to show promise.