Pet Talk

Dr. Arnon Gal, a small-animal internal-medicine specialist, is shown with his cat at the University of Illinois Veterinary Teaching Hospital in Urbana.

So, your cat was diagnosed with hyperthyroidism, and your veterinarian is recommending radioactive iodine. What is it? Is it effective? Is it painful?

Dr. Arnon Gal, a small-animal internal-medicine specialist at the University of Illinois Veterinary Teaching Hospital, describes the conditions for and successes of radioactive iodine treatment for hyperthyroidism.

Hyperthyroidism is a disease of the thyroid gland that causes its hormones to be produced in excess. Although it is often due to a benign tumor in the gland, the causes that lead to the transition from normal to abnormal are not fully understood.

"Current thinking suggests that hyperthyroidism arises from exposure to a combination of factors," Gal said. "These include environmental endocrine-disrupting compounds found in household plastics, an excess of iodine in the cat's diet, and acquired mutations in receptors in the thyroid gland that stimulate the production of thyroid hormones."

Symptoms most often seen are persistent thirst, weight loss despite increased appetite, and hyperactivity or nervousness. Hyperthyroidism is diagnosed by a combination of supporting clinical signs, abnormalities revealed by blood tests, and increased thyroid hormone levels. It is important to consult with your local veterinarian if hyperthyroidism is suspected.

Radioactive iodine treatment is relatively painless. The cat is sedated and a catheter is placed in its vein, through which the radioactive iodine is injected.

When injected into the blood, radioactive iodine (also called I-131) is predominantly taken up by the abnormal part of the thyroid gland that is producing excess hormones and not by the healthy part of the gland. Exposure to I-131 selectively destroys the abnormal parts of the thyroid.

After the cat has gone through the initial hospital isolation, they go home but must be kept separate from other pets and people for 28 days. During this time, their feces and urine from the litter pan will need to be disposed of in bags. Old people, young children, pregnant women and people with certain diseases cannot be in contact with the cat or their secretions during this isolation period.

"At the veterinary teaching Hospital, cats receiving radioactive iodine need to be in isolation for five to seven days. During this time, we can give them food and water but cannot touch them," Gal said. "For that reason, cats that need daily injections, such as insulin, or that require medications that they will not consume voluntarily are not candidates for this treatment. Cats that will not eat on their own also are ineligible."

There are two criteria that must be met in order for a cat to receive radioactive iodine treatment:

"The success rate of a single treatment is about 97 percent," Gal said. "Less than 2 percent of cats will have a recurrence of hyperthyroidism and may need a second treatment."

A small portion of cats might experience decreased thyroid hormone levels. If this occurs, they will need supplementation for an extended period or life.

There are no side effects for I-131. However, because increased thyroid hormone levels increase kidney function, it is expected that all cats will have a slight decrease in kidney function after treatment.

"This is not a problem in most of the cats," Gal said.

In some cats, decreased kidney function becomes evident on routine bloodwork after their treatment with I-131. This does not mean they developed kidney disease after treatment, but rather that it was hidden by the hyperthyroidism and only became clear once their thyroid hormone levels were lowered. These cats can receive medical management like any other with chronic kidney disease.

"It is important to remember that not treating hyperthyroidism actively damages the kidneys," Gal said. "High thyroid hormone levels increase the amount of blood that is filtered by the kidney, which damages specific parts of the kidney that filter blood."

If you have questions about hyperthyroidism and radioactive iodine treatment in cats, consult your local veterinarian.

An archive of pet columns from the UI College of Veterinary Medicine is at vetmed.illinois.edu/petcolumns. Requests for reprints of this article may be directed to Chris Beuoy at [email protected].

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