Radioactive Iodine Treatment for Cats with Hyperthyroidism

INFORMATION AND INSTRUCTIONS FOR OWNERS

What is Hyperthyroidism?

Hyperthyroidism is the condition resulting from secretion of excess thyroid hormone. In cats, this usually results from adenomatous hyperplasia or adenoma (begin tumor) of the thyroid gland. Carcinoma (cancer) is a very rare cause of hyperthyroidism in cats.

How can hyperthyroidism be treated in cats?

Hyperthyroidism can be treated medically, surgically, or with radioiodine. 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). Radioiodine (Radioactive Iodine) will also cure the hyperthyroid condition. The procedure for this treatment is relatively simple, consisting of a single injection administered subcutaneously.

Why would Radioiodine be used instead of medicine or surgery?

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 pill. Second, mild reactions (e.g., loss of appetite, vomiting) are common, whereas a few cats develop serious adverse reactions to the medication (blood or liver problems). Because of these side effects, periodic blood tests are needed to monitor the cat's condition are necessary. Finally, some owners may not want to have to medicate their relatively young cat for the rest of it's life (pills must be given daily).

Surgery is generally an effective treatment for hyperthyroidism but may have disadvantages in some cats. Many cats with hyperthyroidism have heart problems and are higher anesthetic risks. 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.

Radioactive therapy has some distinct advantages over use of medical or surgical treatment. With radioiodine, 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 at least one week before radioactive iodine is given. The major drawback is that after administration of radioiodine, the cat must be kept hospitalized for a period (usually one to two weeks).

How does radioiodine 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 may lead to death.

Radioiodine-131 (131I) 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 consists of two types: gamma rays (similar to X-rays) and beta particles (electrons). The half-life of 131I is 8 days; in other words, one-half of the radioiodine goes through this process every 8 days.

When injected into the body, a large percentage of the radioiodine accumulates in the thyroid gland. The remainder of the 131I is excreted primarily in the urine and to a lesser degree in the feces. Once the radioiodine is taken up by the thyroid gland, (or thyroid tumor in a hyperthyroid cat) the gamma rays and beta particle's are released. The beta particles are particularly lethal to the thyroid cells. The gamma rays effect the thyroid but may also escape the cat's body. The radiation destroys the thyroid tumor cells and thus treats the hyperthyroid condition.

How is therapy given?

The radioiodine is given as a single injection (subcutaneously) on the first or second 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. Your cat is then monitored over the next one to two weeks until the cat's radioactivity level is low enough to permit his/her return to you.

How long is the hospitalization period?

The half-life of the iodine used is 8 days. The iodine is excreted primarily in the cat's urine. Since each cat's metabolism is slightly different, the length of stay varies and is determined by measuring the degree of radioactivity over your cat's thyroid gland. The usual period is from one to two weeks (average 10 days).

How can I check on my cat's condition during the treatment? Is visiting allowed?

Due to the nature of the treatment, visitation is not permitted. The cat's are fed and monitored two to three times daily by a full time veterinary technician. We generally call you with a progress report every day during the work week. If you wish you can also call us to check on the status of your cat.

Are there any side effects or risks of therapy?

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 (cobalt radiation). Some cats seem to experience mild discomfort of the thyroid region (thyroiditis) at the beginning of therapy, but this resolves itself spontaneously and does not cause a problem. Occasionally, a cat will develop hypothyroidism (under active thyroid gland) after treatment with radioiodine. This is easily controlled with supplementation and may not be permanent. Overall, side effects are extremely rare.

How do I make and appointment for this treatment?

Please ask your veterinarian to send us (via fax or mail) the result of your cat's history, physical examination, and recent blood work (e.g., complete blood count, biochemical profile, urinalysis, and T4 test) so that Dr. Peterson can evaluate if your cat is a good candidate for treatment. We need blood tests taken within 30 days of your appointment for radioiodine treatment. If this data is acceptable, we will than schedule an appointment for treatment. If you do not hear from us within a day or two after we have received your cat's records, please do not hesitate to call Dr. Peterson's office directly to schedule an appointment.

What do I do on the day of treatment?

Please bring your cat to the hospital at the scheduled time. You should feed your cat on the day of admission (fasting is not necessary). Your veterinarian should have done all or the majority of tests necessary before treatment; if further tests are required, they an be performed by us, but then treatment may have to be postponed. If your cat has been on methimazole (Tapazole), these drugs must be discontinued for at least one week prior to treatment.

Should I bring anything for my cat?

If your cat eats a particular kind of food, we suggest that you bring a few cans so we know exactly what you want your cat to eat during the time of hospitalization. If you want to bring one or more small personal items (a sock or toy) to place in your cat's cage, we do not allow it. However, if you do, please understand that we cannot return the personal items because they will likely become contaminated.

What will happen on the day of admission?

We will examine and collect blood from your cat in order to repeat the thyroid profile. If all other routine blood tests have been done by your veterinarian we will treat your cat on the day of admission.

Is Radioiodine therapy expensive?

The cost of any treatment for feline hyperthyroidism is significant. The cost for radioiodine therapy is generally a total fee of $1,100 (as of 1998), but may be more if the hyperthyroidism is severe. This includes: the radioiodine itself, the cost of hospitalization, food, liter, and motoring. Tests required prior to therapy are not included. This compares reasonably with most fees for surgical removal. The cost of medication would average $275 (as of 1998-generic medications now available) per year plus the cost of repeat blood tests. Please note that, because we must order the dose of radioiodine the day before the treatment is administered, cancellation of a scheduled appointment (less than 24 hours of the appointment time) will result in a charge of $200.

How do I pay for Radioiodine Therapy?

Payment for consultation and tests should be made at the time of service. Payment for Radioiodine therapy requires a deposit of one-half the total bill with the remainder due upon your pet's discharge. As far as method of payment is concerned, please note that credit cards will not be accepted. Payment must be made in the form of personal check or cash.

MARK E. PETERSON ,DVM, DIPLOMATE ACVIM THE ANIMAL MEDICAL CENTER 5120 EAST 62ND STREET NEW YORK, NEW YORK 10021 212-838-8100, EXT. 213/ 212-486-1699 (FAX)