Guide to Radioiodine Treatment for Feline Hyperthyroidism at
Oradell Animal Hospital

WHAT IS HYPERTHYROIDISM?

Hyperthyroidism is a clinical condition resulting from the excessive production of thyroid hormones by the thyroid gland. It is common in middle-age to older cats. In fact, greater than 95% of the cases occur in cats over 8 years of age. It is usually due to benign changes (hyperplasia or adenomas) in the thyroid gland. Cancer of the thyroid gland can occur but it is rare.

WHAT ARE SIGNS OF HYPERTHYROIDISM?

Thyroid hormone affects every organ system, so signs can be variable. The most common clinical signs include weight loss, increased appetite, vomiting, diarrhea, drinking and urinating more, and nervousness or hyperactivity. Thyroid hormone affects the heart, causing fast heart rates, heart murmurs, abnormal heart beats, and high blood pressure.

HOW CAN HYPERTHYROIDISM BE TREATED?

Three forms of therapy are available. These include:

  1. Anti-thyroid drugs such as methimazole (Tapazole®) or ipodate.
  2. Surgical removal of the thyroid glands.
  3. Radioactive iodine (I131).

HOW DOES RADIOACTIVE IODINE WORK?

Iodine is normally taken up by the thyroid gland. One form of iodine, I131, is radioactive. When I131 enters the thyroid gland it destroys the abnormally functioning cells. This reduces the size of the gland and its ability to produce thyroid hormone. I131 is administered to hyperthyroid cats by a subcutaneous injection. Cats treated with radioactive iodine need to be hospitalized for 4-7 days following the injection. This is when they are most radioactive. Their level of radioactivity is checked daily with a Geiger counter and only when it reaches an acceptably low level can they go home.

HOW EFFECTIVE IS RADIOIODINE THERAPY FOR FELINE HYPERTHYROIDISM?

In a recent study of 524 hyperthyroid cats treated with radioiodine, the overall response to treatment was considered good in 94% of the cats. 8/524 (1.5%) remained hyperthyroid 6 months following treatment, requiring additional therapy. 13/524 (2.5%) had a relapse of hyperthyroidism 1-6.5 years after the initial treatment.

IS RADIOIODINE TREATMENT SAFE?

Radioiodine treatment has been used in human medicine for over 50 years and is recognized as a safe and effective method of treating human patients with hyperthyroidism. Studies in humans have shown no increased risk of developing leukemia, thyroid cancer, or other cancers after treatment with radioiodine. In a recent study of 524 hyperthyroid cats treated with radioiodine, the only adverse sign was problems swallowing in 8/524 (1.5%) cats. This problem resolved rapidly without treatment.

WHAT ARE THE RISKS OF MY CAT BECOMING HYPOTHYROID?

Hypothyroidism (the opposite of hyperthyroidism) is a condition resulting from a deficiency of thyroid hormones. Clinical signs include lethargy, dullness, obesity, oily skin, and matted fur. In a recent study of 524 hyperthyroid cats treated with radioiodine, only 11/524 (2.1%) developed hypothyroidism and required thyroid hormone supplementation.

IS THERE ANY INCREASED RISK IN TREATING MY CAT FOR HYPERTHRYODISM IF THERE ARE OTHER HEALTH PROBLEMS LIKE CHRONIC KIDNEY DISEASE?

Because hyperthyroidism is seen in middle-age to older cats and may affect any organ system, it is not unusual for a hyperthyroid cat to have other health problems. Chronic kidney disease and heart disease are important concerns in these patients. Therefore, careful evaluation of hyperthyroid cats is recommended. A complete blood workup, including tests evaluating kidney and liver function, a urinalysis, a thoracic radiograph to evaluate the heart and check for cancer, and blood pressure measurement are performed in the initial evaluation of hyperthyroid cats prior to treatment. These tests help give a better understanding of each individual hyperthyroid patient’s specific needs.

WHAT ARE THE RISKS OF RADIATION EXPOSURE FROM MY CAT TO PEOPLE AND OTHER PETS?

The dose of radioiodine used to treat hyperthyroid cats is very small and people and other animals are at very low risk from the radiation. Treated cats are hospitalized until they reach an acceptably low level of radioactivity. However, they will be radioactive (to some degree) for 82 days following treatment. Radioactivity does decrease rapidly over time and distance.

STEPS TO MINIMIZE ANY UNNECESSARY RADIATION EXPOSURE TO OTHERS

  1. For the first 2-3 weeks, try to maintain an arm’s length distance (~3-6 feet) from your treated cat whenever possible and especially if you will be with it for long periods of time. Avoid sleeping with your cat for the first 2-3 weeks.
  2. Wash your hands carefully after handling your cat, its food dishes, or litter pan.
  3. A treated cat will excrete low levels of radioactive iodine in its urine for several weeks. Wear disposable plastic gloves when changing the litter and disposable plastic litter pan liners to minimize handling the litter. Change the litter daily and try to prevent tracking of litter away from the box. Place all contents in a plastic bag and keep it out of the normal trash (store outside your home) for 3 weeks, and then you can dispose of it with your regular trash.
  4. Keep your treated cat confined indoors for 82 days. Do not allow your cat to go outside and roam freely in the neighborhood.
  5. Children under the age of 18 years and pregnant women should not have any prolonged or close contact with your treated cat.
  6. Contact us with any problems or questions regarding the treatment and home care of your cat.

PLEASE NOTE: ONCE WE HAVE SCHEDULED THE I-131 TREATMENT, WE WILL ORDER THE I-131. THIS ORDER CAN ONLY BE CANCELLED BY THE FRIDAY BEFORE THE SCHEDULED TREATMENT.

Treatment Options

Medical treatment consists of giving oral medication two or three times daily for the remainder of the cat’s life. The medication does not cure the disease; it merely controls the release of thyroid hormone. There is an incidence of side effects such as vomiting, diarrhea, poor appetite, and skin lesions. Missed medication will result in relapses. Surgical removal of the affected thyroid gland(s) will cure the disease, however, there are risks of anesthesia and surgical complications in older and often frail cats.

Radioactive iodine (I-131) therapy is the treatment of choice, because it is safe and effective.
With just one subcutaneous injection and hospitalization for four days, 98% of affected cats are cured of the disease. Our staff is on site 24 hours a day monitoring the patients in our large, sunny Nuclear Medicine Ward.

Baci
Casey
Baci underwent I-131 treatment and is doing so well. Everyone at Oradell Animal Hospital was wonderful and took great care of him! Casey Flora, a patient of Dr. Laura Eirmann, underwent 1-131 treatment for hyperthyroidism. Before his treatment he had an excessive appetite and although he was eating well he was losing a lot of weight. Since his therapy, he is happy and at his normal healthy weight.
Mickey and Gizzie
Alaska
Mickey, on the left, is happy to have Gizzie home. Gizzie just underwent I-131 treatment and is doing great! Gizzie says thanks to Dr. Shah. Before Alaska’s I-131 treatment, she was very skinny and her eyes looked like they were wide open all the time. She had a funny smell and was shedding more than usual. Since her treatment she has gained her weight back, her eyes look normal and her coat is much fuller.

Laura Sartor, DVM, DACVIM (Internal Medicine)

Dr. Sartor graduated from the Atlantic Veterinary College in Prince Edward Island. She completed a rotating internship in small animal medicine and surgery at the Western College of Veterinary Medicine in Saskatchewan and a residency in small animal internal medicine at the University of Wisconsin Veterinary Hospital in Madison. Dr. Sartor is board certified by the American College of Veterinary Internal Medicine, and is in charge of Oradell Animal Hospital's Nuclear Medicine Program utilizing radioiodine for the treatment of hyperthyroidism in cats. Her special interests also include liver disease, coagulation disorders and anemias. Dr. Sartor enjoys traveling and shopping.