Radioactive Iodine (i131) Therapy For Hyperthyroidism In Cats

Hyperthyroidism

What is hyperthyroidism?

Hyperthyroidism is a common disease of middle-aged and geriatric cats. The signs of hyperthyroidism are due to excessive thyroid hormone production. Ninety eight percent of cases of hyperthyroidism are due to a benign thyroid tumor (adenoma) or excessive thyroid gland growth (hyperplasia). Malignant thyroid tumors are rare and account for only 1-2% of hyperthyroid cats.

What are the signs of hyperthyroidism?

Common signs of hyperthyroidism include weight loss despite a good appetite, excessive water consumption and urination, hyperactivity, vomiting and diarrhea. More serious signs including heart failure, hypertension and eventually death may develop if hyperthyroidism is not treated.

How is it treated?

Treatment options for hyperthyroidism include daily oral medication (e.g. Tapazole), surgical removal of one or both thyroid glands, and destruction of diseased thyroid tissue with radioactive iodine (radioiodine). Radioiodine is considered the treatment of choice in the majority of cats with hyperthyroidism.

Radioiodine Therapy

What does the treatment process involve?

Radioiodine therapy is performed on-site at Vermont Veterinary Internal Medicine (VVIM).

Radioiodine is administered as a single dose under the skin (subcutaneously) just like when your cat is vaccinated. Radioiodine is concentrated exclusively by active thyroid tissue. The activity of “normal” thyroid tissue has been suppressed by the excessive amount of hormone produced by diseased tissue. Therefore, normal thyroid tissue and other organs do not accumulate radioiodine and are generally not affected by therapy. The local emission of radioactive particles destroys the diseased thyroid tissue. Thyroid hormone concentrations usually return to normal within one month of radioiodine therapy.

Treated cats and their excretions will be temporarily radioactive. In an effort to limit radiation exposure to our clients and the environment, VVIM requires that our patients remain hospitalized a minimum of 96 hours after treatment or until they are emitting no more than 0.5 mR/hr measured at one foot. Radiation levels are monitored daily beginning on the third day after treatment. Most cats can be released within 5-8 days. During this “cooling off” period, cats will be individually housed in VVIM’s radiation isolation ward.

Radiation safety regulations require that direct contact with hospitalized radioiodine patients be kept to a minimum. A webcam is used to monitor our patients 24 hours a day. VVIM technicians clean the cages, and feed and water your pet at least twice daily. The main room lights are turned off at night and a radio is played during the day to minimize the stress of hospitalization. Personal effects are welcome; however, we cannot return these items due to possible radioactive contamination.

Can I visit my cat?

Without exception, direct owner visitation is not allowed while cats are in isolation. However, you are welcome to virtually visit your cat via the internet by clicking on the link to the suite number you were given at check in. If you do not have internet access, we are happy to show you the video images on a monitor in the main hospital treatment area.

Is it safe to my cat?

Adverse side effects of radioiodine therapy are rare. Less than 5% of hyperthyroid cats will become symptomatic for hypothyroidism (low thyroid hormone concentration) after radioiodine therapy due to destruction of all thyroid tissue. Signs of hypothyroidism include lethargy, obesity, dry flaky skin and matting of hair. This is not typically life threatening but may require indefinite oral thyroid hormone supplementation. Less than 2% of treated cats develop signs of inflammation of the thyroid glands (thyroiditis). Thyroiditis may cause a mild fever, transient loss of appetite or difficulty swallowing. Signs of thyroiditis usually resolve without treatment before the cat is released from the hospital.

If a cat were to become ill during the “cooling off” period after radioiodine therapy, radiation safety regulations limit what can be performed diagnostically (e.g. blood testing) and therapeutically. Therefore, it is very important that any other pre-existing disease has been identified and the cat is eating well prior to radioiodine therapy. VVIM recommends that complete blood testing (CBC and chemistry profile), urinalysis and chest and abdominal radiographs are evaluated within 3 weeks of radioiodine therapy. Generally, your referring veterinarian will perform this testing.

Is it safe to my family?

When cats treated with radioiodine are released from the hospital they are emitting no greater than “background” radiation levels. A person standing one meter (approximately 3 feet) from the cat is exposed to radiation levels comparable to that received when standing outside on a sunny day. However close contact (i.e. less than a meter) with the cat may result in slightly higher than background radiation until residual radioiodine has more completely decomposed.

VVIM recommends that clients take the following precautions for the 2-week period following release:

  • The pet should be kept indoors or in an enclosed area.
  • Avoid prolonged physical contact with the animal (e.g. do not let it sleep in bed with you).
  • Always wash your hands after contact with your pet.
  • Maintain a litter pan for the animal’s urination and defecation. Soiled litter should be disposed of daily in a plastic bag. The plastic bag should be placed in an outside disposal. You should wear rubber or latex gloves when cleaning the litter box and disposing of litter.
  • Children less than 18 years of age or pregnant women should have no contact with the pet.

You may resume normal activities after the two-week period.

Is it effective?

Approximately 90-95% of cats with hyperthyroidism due to thyroid adenoma or hyperplasia are cured by a single treatment with radioiodine. The remainder of these cats requires a second treatment to be cured. Approximately 1-2% of cats with hyperthyroidism have a malignant thyroid tumor (carcinoma) and may not be cured with standard doses of radioiodine.

What is the cost?

$995.25* (This includes the initial consultation visit, radioiodine treatment and hospitalization but does not include the recommended pre-treatment or follow-up testing. Your referring veterinarian generally performs this testing).

If you have any questions about hyperthyroidism or radioiodine therapy for your cat, please contact Dr. Bryan Harnett at VVIM at (802) 863-2387.

* VVIM reserves the right to change fees at any time without notice.