Ask Elizabeth

January 2019 Issue




Consider Radioiodine Therapy

Long-term methimazole can become less effective

Sox the cat

The treatment Sox has been receiving for hyperthyroidism may have stopped working.

Q. We have a 17-year-old Tuxedo cat, Sox, who is still active, sharp-witted, and good natured. However, he has hyperthyroidism, and while he is being treated with methimazole and was doing fairly well, he has recently begun to lose weight again. In his prime years, he was a healthy 10-pound cat. Now he is down to six or maybe fewer pounds and looks boney, although he eats throughout the day and gets plenty of treats.  Can you provide any advice?

A. Thanks for getting in touch, and I am sorry, but not surprised, to hear that your baby has hyperthyroidism. This condition is quite common in older cats, and while it can certainly negatively impact the lives of affected cats, the good news is that there are effective therapies to treat it. Perhaps a brief discussion of hyperthyroidism would be helpful.

Hyperthyroidism results from the overproduction of thyroid hormone (which is vital to the normal function of many organs in the body) by the thyroid gland, usually due to the development of a benign adenoma (non-malignant tumor) of this gland. The cat’s thyroid gland sits in the neck near his “vocal chords.” In most cases of feline hyperthyroidism, the enlarged thyroid gland can be felt upon careful examination of this region. In rare cases, malignant tumors of the thyroid gland occur, but this is only found in approximately 2 percent of cats with hyperthyroidism.

Cats with hyperthyroidism generally show signs such as hyperactivity (they may also vocalize more and at strange times of the day), weight loss, increased appetite, increased water intake and urination, vomiting, and an unkempt haircoat.

The diagnosis of hyperthyroidism is made via blood tests that usually measure the amount of thyroid hormone (called thyroxine) in the bloodstream. There are some supplemental tests—including imaging studies of the thyroid gland using radioisotopes—that can be done in some cases.

There are generally four treatment options for hyperthyroidism in cats:

-Surgical removal of the thyroid gland. This procedure is almost never elected anymore due to safer, more effective options.

-Methimazole (the drug you are using) is an oral drug that blocks the production of thyroxine in the thyroid gland. In some cases of long-term use, however, it can become less effective, as the thyroid adenoma usually continues to grow.

-Feeding a diet that is low in iodine. Iodine is essential for the production of thyroxine. While the long-term effects of this diet are the subject of debate, they can be used successfully, but it’s important that a cat on this diet to treat hyperthyroidism not be fed ANY other foods.

-Radioiodine therapy. This option is perhaps the best. It is one that is very effective, has few side effects when appropriately administered, and cures cats of hyperthyroidism approximately 95 percent of the time that it’s used.

This therapy involves injecting a radioactive compound that is linked to iodine, which is taken up by the thyroid gland as it produces thyroxine. Once this iodine-tagged isotope is in the thyroid gland, the radiation that it emits destroys overactive thyroid gland tissue in a very localized and usually safe manner, resulting in more normalized thyroxine production.

In rare cases, cats may become hypothyroid and require thyroxine supplementation or may require a follow-up dose of radioiodine to successfully restore normal thyroxine levels, but these are highly unlikely to occur.

I recommend that you consult with your veterinarian about Sox. It may be that his hyperthyroidism is not well controlled on his current dose of methimazole and that he either needs a higher dose or another treatment modality (like radioiodine therapy). It also may be that his recent weight loss has nothing to do with his thyroid function and is due to another problem, such as diabetes mellitus, kidney disease, gastrointestinal disease, or a variety of other conditions that can cause this clinical sign.

I hope that this is helpful, and we wish you the best of luck. Please drop us an update on Sox when you can.

All My Best,

Elizabeth