Radioactive Iodine for Hyperthyroid Cats

a cat looking out the window

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.

What Is Hyperthyroidism?

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

Dr. Arnon Gal is boarded in both small animal internal medicine and veterinary pathology (anatomic).

“Current thinking suggests that hyperthyroidism arises from exposure to a combination of factors. 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,” says Dr. Gal.

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.

What Makes a Good Candidate for Radioactive Iodine?

“At the Veterinary Teaching Hospital, cats receiving radioactive iodine need to be in isolation for 5 to 7 days. During this time, we can give them food and water but cannot touch them,” says Dr. Gal. “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 cat does not need medications that have to be physically administered by hospital staff.
  • The cat’s owners do not have underlying conditions that prevent them from managing the cat at home in isolation for 28 days while disposing of the cat’s excretions as instructed.

What Is Radioactive Iodine?

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 thyroid hormones and not by the healthy part of the gland. Exposure to I-131 selectively destroys the abnormal parts of the thyroid gland.

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. For that reason, I-131 treatment may not be suitable for every hyperthyroid cat.

Effectiveness of Radioactive Iodine

“The success rate of a single treatment is about 97%,” says Dr. Gal. “Less than 2% of cats will have a recurrence of hyperthyroidism and may need a second treatment.”

A small portion of cats might become hypothyroid, which means that they will have decreased thyroid hormone levels. If this occurs, they will need thyroid hormone supplementation for an extended period or for 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,” says Dr. Gal. “It becomes a problem in cats that develop hypothyroidism and necessitates supplementation with thyroid hormones.”

Hyperthyroidism and Kidney Disease

In some cats, decreased kidney function becomes evident on routine bloodwork after their treatment with I-131. This finding does not mean they developed kidney disease after treatment, but rather that their kidney disease was unmasked after their thyroid hormone levels were lowered. These cats can receive medical management like any other cat with chronic kidney disease.

“It is important to remember that not treating hyperthyroidism actively damages the kidneys,” says Dr. Gal. “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.
By Crystal Munguia

Featured photo by Lesli Whitecotton on Unsplash