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Radioactive Iodine Therapy for Feline Hyperthyroidism

What is hyperthyroidism?

Hyperthyroidism is a relatively common disease of older cats caused by the excess production of thyroid hormone. In most cases it is caused by a benign tumor-like growth of the thyroid gland. In up to 4 percent of cats, however, it is caused by a malignant thyroid cancer, which can spread to other body regions.

What are the signs of hyperthyroidism?

Excess thyroid hormone levels in the body can cause a variety of signs including weight loss, voracious appetite, poor haircoat, increased thirst and urination, vomiting and diarrhea, and hyperactivity. The disease can also cause high blood pressure and damage the heart. If left untreated, the signs of hyperthyroidism will progress eventually causing death.

How is hyperthyroidism diagnosed?

Hyperthyroidism is diagnosed by finding elevated thyroid hormone levels on blood tests. Technetium scanning of the thyroid gland is then performed prior to treatment to confirm the presence of hyperfunctioning thyroid glands as well as to discern their benign or malignant nature.

How is hyperthyroidism treated?

Hyperthyroidism can be treated by 1) the lifelong daily administration of tablets, 2) surgical excision of the thyroid glands, or 3) the administration of a single injection of radioactive iodine. Each treatment option has advantages and disadvantages. The oral tablets are relatively inexpensive but do not cure the condition, must be given twice daily indefinitely, and can cause serious side effects. Surgical removal of the thyroid glands can cure the benign form of the disease but requires anesthesia and risks damage to nerves and nearby calcium regulating glands in the neck region. An injection of radioactive iodine, meanwhile, will cure 97% of hyperthyroid cats and has rare side effects, but state laws require that treated cats be hospitalized in quarantine until their radioactivity decreases to regulation levels. Radioactive iodine is considered to be the safest and most effective treatment for hyperthyroidism.

How does radioactive iodine treatment work?

Iodine is used by the thyroid gland to produce thyroid hormone. The thyroid gland cannot differentiate normal iodine from radioactive iodine. Radioactive iodine is injected intravenously into your cat's body and taken up by the thyroid gland where it kills the hyperactive tumor cells. Because only the thyroid gland utilizes iodine, other parts of the body are not damaged.

People with hyperthyroidism (Graves Disease) are also treated with radioactive iodine and are immediately sent home. Unfortunately, we cannot simply tell our cats to stay away from children and to flush the toilet twice after urinating (most of the radioactivity is excreted from the body in the urine). Consequently, state laws were passed that require treated cats to be hospitalized in quarantine for approximately 7 days until the radioactivity in their body has decreased to desired levels. Only specially trained personnel are allowed to handle treated cats during this quarantine period.

Is radioactive iodine treatment safe?

The radioactive iodine injection is very safe. However, because cats must be hospitalized away from home after treatment, only those in otherwise good health can be treated in this manner. Mild sedation is required to perform the technetium scan but is of little risk.

Rarely, cats may develop excessively low thyroid hormone levels after treatment. Signs of low thyroid hormone levels include lethargy, obesity, and flaky dry skin. Oral thyroid supplements are used to treat the condition.

Less than three percent of treated cats may still have elevated thyroid levels 3 months after treatment. These cats require a second dose of radioactive iodine to achieve a cure.

Malignant thyroid cancer can cause tumors to develop in other body regions and requires higher radioactive iodine doses to treat. Its presence is determined prior to treatment with a technetium scan.

Resolution of the hypermetabolic state caused by hyperthyroidism, can make pre-existing kidney disease more obvious after treatment.


How is radioactive iodine treatment done?

Because of state regulations regarding the use of radioactive iodine, we are presently treating hyperthyroid cats at the University of Kansas Medical Center (KUMC) in cooperation with their veterinary and technical staff. Hyperthyroid cats are initially evaluated at Veterinary Specialists of Kansas City. Those cats in satisfactory health are then transported to the Medical Center by Dr. Dennis. Before radioactive iodine treatment, a technetium scan is performed to confirm hyperthyroidism and identify the extent of disease (malignant vs. benign). Cats with the benign form of hyperthyroidism are then given an intravenous injection of radioactive iodine and quarantined in a special observation room. Cats are evaluated daily by the KUMC staff responsible for their care. Their observations are reported to Dr. Dennis and the staff at Veterinary Specialists of Kansas City who in turn will maintain daily phone contact with you. Once radioactivity levels have decreased to regulation levels (usually 7 days), your cat is transferred back to Veterinary Specialists of Kansas City for discharge.

After treatment, what precautions should you take at home?

Your cat will be passing very small amounts of radioactive iodine in the urine for another 10 days. While your pet poses no significant health risk to you or your family, the following recommendations should be adhered to in order to insure compliance with University, State, and Federal regulations. To avoid unnecessary human exposure, we recommend for the 10 days following discharge that:

1. Your cat be kept indoors.

2. Disposable gloves are worn when changing the litterbox. Litter should be disposed of daily in an outside plastic bag.

3. Hands are washed after the cat, its food dishes, or the litterbox is handled.

4. The handling of your cat by children be minimized.

5. Family members not sleep with the cat.

6. Have your veterinarian check thyroid hormone and kidney blood values three weeks after treatment is completed


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