- RADIOACTIVE IODINE (I-131) THERAPY FOR HYPERTHYROID CATS -
- INFORMATION FOR CAT OWNERS -

WHAT IS HYPERTHYROIDISM?
Hyperthyroidism is the condition resulting from secretion of excess thyroid hormone. In cats, this usually results from adenomatous hyperplasia or an adenoma (begin tumor) of the thyroid gland. Carcinoma (malignant cancer) of the thyroid gland is a very rare cause of hyperthyroidism in cats. We do not know why cats develop thyroid disease, but it is a disease seen in cats over 4 years of age and most of our patients are 10-16 years old.

WHAT ARE SIGNS OF HYPERTHYROIDISM?
The typical hyperthyroid cat is a pet that is losing weight but still eating normally, or eating more than normal. Other signs include a rough or greasy appearance to the hair coat, behavior changes (such as hyperactivity or aggression), heart murmur and heart enlargement, increased liver enzymes, increased drinking and urinating, vomiting, diarrhea, and/or panting. A small number of cats with hyperthyroidism will have a loss of appetite and signs of lethargy in addition to other signs (sometimes called apathetic hyperthyroid cats). Blood tests should confirm elevated thyroid hormone levels. Your veterinarian can usually feel a small lump in your cat’s neck when abnormal thyroid tissue is present. However, normal and abnormal thyroid tissue can be anywhere from the base of the tongue to the base of the heart, and when it is inside the chest it cannot be felt or located except with specialized nuclear imaging studies (scintigraphy – see images at the bottom of this page).

WHAT TREATMENT OPTIONS EXIST?
Hyperthyroidism can be treated medically, surgically, or with radioactive iodine. Medical treatment consists of administration of methimazole (Tapazole) one to three times per day. Methimazole treatment will usually control hyperthyroidism but is not a cure (i.e., the drug will block thyroid hormone secretion but will not remove the thyroid tumor), and the drug must be given for the rest of the cat's life. Surgery will cure the hyperthyroid condition and consists of the removal of part or all of the thyroid gland (the feline thyroid gland consists of two separate lobes). Radioactive Iodine (I-131) will also cure the hyperthyroid condition. This treatment is relatively simple, consisting of a single injection administered intravenously or subcutaneously.

WHY IS RADIOACTIVE IODINE BETTER THAN SURGERY OR MEDICATIONS?
Medical therapy will work for the majority of cats, but there can be several reasons it may not be the best choice. First some cats may be difficult or impossible to administer medications to. Second, mild reactions (e.g., loss of appetite, vomiting) are common, and some cats develop serious adverse reactions to the medication (blood or liver problems). Periodic blood tests are needed to monitor the cat's condition, because of these side effects. Finally, some cat owners may simply not wish to medicate their relatively young cat daily for the rest of it's life (Tapazole must be given multiple times daily).
Surgery is generally an effective treatment for hyperthyroidism but has disadvantages in some cats. Cats with hyperthyroidism have systemic metabolic disease including heart problems and have a higher risk of death during anesthesia. There is also a risk that there will be temporary damage to the adjacent parathyroid glands during thyroid surgery resulting in hypocalcemia (low blood calcium). This complication can be life threatening and can result in extra hospitalization and cost. After surgery some cats will develop hypothyroidism and will require treatment with thyroid hormone pills for a period of time. Finally, there is a small risk that the hyperthyroidism will not be cured with surgery, or the condition will reoccur (approximately 10% recurrence rate).
Radioactive iodine therapy has some distinct advantages over use of medical or surgical treatment. With I-131, the need for anesthesia and the risk of hypocalcemia (the major disadvantages with surgery) are eliminated. Tapazole treatment is not needed; in fact, drug therapy must be discontinued before radioactive iodine is given. The major drawback is that after administration of radioiodine, the cat must be kept hospitalized for a period of time(usually 4 days), and special home care is required for 3weeks after release from the hospital to comply with radiation safety requirements.

HOW DOES I-131 THERAPY WORK?
Iodine is an element required for normal health. In the body, it is used primarily by the thyroid gland (located in the neck) to produce the thyroid hormones (T4 and T3 are the two major thyroid hormones). The major effect of thyroid hormone is regulation of metabolic rate. Although essential for life, excess thyroid hormone is toxic to many organs, including the heart and liver. Untreated hyperthyroidism will lead to a wasting syndrome and death.
Radioactive iodine (I-131) is a form of iodine that has been made radioactive. In it's radioactive state, it undergoes a natural process (decay) in which it gives off radiation. The radiation given off consists of two types: gamma rays (similar to X-rays) and beta particles (electrons). The half-life of I-131 is 8 days; in other words, one-half of the radioactive iodine decays to stable iodine every 8 days.
When injected into the body, a large percentage of the radioactive iodine accumulates in the thyroid gland. The remainder of the I-131 is excreted primarily in the urine and to a lesser degree in the feces. Once the radioiodine is taken up by the abnormal thyroid gland, (or thyroid tumor in a hyperthyroid cat) the beta particles released destroy the abnormal thyroid cells. The gamma rays impact the thyroid but may also travel outside the cat's body. The concentrated, local radiation destroys the thyroid tumor cells and thus cures the hyperthyroid condition.

HOW IS I-131 GIVEN?
The radioiodine is given as a single injection (subcutaneously) on the first hospital day. After the treatment is administered, your cat is placed in isolation. In this case isolation means keeping your cat in a separate cage and room, away from other animals not receiving this treatment and limiting human exposure. Urine and feces produced by your cat must be collected and stored at our facility during the first 4 days after iodine injection. Your cat is monitored and released from isolation when the cat's radioactivity level is low enough to permit his/her safe return to you.

HOW LONG WILL MY PET BE HOSPITALIZED?
The physical half-life of the iodine used is 8 days. The iodine is also excreted, primarily in a cat's urine. The hospitalization period is usually 4 days. As each cat's metabolism is slightly different however, a cat is not released until our radiation detecting instruments confirm the pet has reached a safe level. You have the option of keeping your cat with us longer than the minimum time, if you do not want to or cannot comply with standard radiation safety requirements at home.

CAN I VISIT MY CAT IN ISOLATION?
Due to the high radiation levels in isolation, visitation is not permitted. The cat's are fed and monitored by specially trained and experienced veterinary technicians and veterinarians. A staff member will provide a progress report to you daily during your cat’s stay.

ARE THERE ANY SIDE EFFECTS FROM I-131 TREATMENT?
Since the iodine is specific in it's site of action, there is no hair loss or increase in skin pigmentation, as may been seen with other forms of radiation therapy (external beam radiation). Occasionally, a cat will develop hypothyroidism (under active thyroid gland) after treatment with radioiodine. This is rare, easily controlled with supplementation and may not be permanent. Overall, side effects are extremely rare. 2-3% of cats may be resistant to a single injection or I-131 and may require a second treatment for cure, because of a malignant thyroid condition or saturation of the thyroid with non-radioactive iodine prior to treatment.

ARE SPECIAL PRECAUTIONS REQUIRED BECAUSE MY CAT WILL BE RADIOACTIVE AFTER TREATMENT?
Radioactive iodine is very safe, but your cat will have a low level of radioactive material in its body when it goes home. To avoid any human radiation exposure from your cat we advise you limit contact with your cat by not allowing your cat to sleep in a bed or rest on a person’s lap, and advise you to spend only the time necessary for basic pet care with your cat during the first 3 weeks after release. You are also advised to use rubber gloves when changing the litter box during this time. Pregnant women and young children should not spend time with your cat during the first 3 weeks after treatment.
To put the radiation exposure you can get from your cat in perspective, the average person is exposed to about 300mrem/year of radiation from natural sources (the sun, soil, medial x-rays, television and computer screens etc.). If you spend 3 hours per day in close contact with your cat after treatment, you will increase your yearly exposure by approximately 3mrem, or a 1% increase for the year. (Based on estimates from the Union of Concerned Scientists, this amount of radiation exposure represents a hazard similar to you drinking one cup of coffee or diet soda daily during this time period.)

DO I NEED TO KEEP MY OTHER PETS AWAY FROM MY CAT WHEN HE/SHE IS DISCHARGED FROM THE HOSPITAL?
No. Cats that receive the full dose of iodine from treatment do not have any higher rate of cancer or other adverse effects that might be linked to radiation exposure, and there is no evidence to suggest other pets sharing the environment are at any risk. Our radiation safety guidlines are designed to keep human exposure to a negligible level, because it is assumed a person will live 70-90 years and be exposed to other radiation through their own medical procedures during this time, and our goal is not to contribute to this other exposure.

HOW DO I MAKE AN APPOINTMENT FOR TREATMENT?
Please ask your veterinarian to send us (via fax 973 833-0381) your cat's recent medical history, physical examination findings, and recent blood work so that our radiologist can confirm that your cat is a good candidate for treatment. If the radiologist confirms with your veterinarian that your pet is a good candidate for radioiodine therapy, we will schedule an appointment for treatment. If you would like additional information or to schedule an appointment call 973-641-3185.

WHAT MEDICAL TESTS ARE RECOMMENDED PRIOR TO TREATMENT?
A complete blood count, biochemical profile, urinalysis, and T4 test are recommended within 30 days of your appointment for radioiodine treatment. Chest x-rays taken within the last 3 months are also valuable to screen your pet for any serious underlying disease that might make a stay in isolation dangerous, or worsen your pet’s prognosis. Occasionally other tests may be suggested.

SHOULD I STOP TAPAZOLE/METHIMAZOLE PRIOR TO MY APPOINTMENT?
If your pet is already on Tapazole, we recommend you stop Tapazole 7-14 days before your appointment to ensure the maximum uptake and effect of the I-131.

WHERE IS YOUR FACILITY LOCATED?
Our therapy facility is at the Animal Emergency and Referral Associates (AERA), at 1237 Bloomfield Avenue, in Fairfield, NJ 07004 (main desk tel. 973-226-3282). For directions to the AERA click this link-http://www.animalerc.com/patients/contact/contact.shtml