Radioactive Iodine 131 Therapy

Feline hyperthyroidism (producing too much thyroid hormone) is recognized as the most common endocrine disorder in the cat. In 97% of feline hyperthyroid cases, the cause is a benign tumor of the thyroid gland that produces excess thyroid hormone, resulting in multi-systemic disease. The metabolic rate of the body is dramatically increased, making the body work much harder than normal. In the process, hyperthyroidism potentially masks certain underlying diseases like kidney failure. Clinical signs of hyperthyroidism vary from mild in the early stages, to severe or life threatening signs in the final stages. A thorough medical work-up is necessary to determine if the changes (weight loss, behavioral changes, ravenous appetite, vocalization, hyperactivity, rapid heart rate and/or murmurs, intermittent vomiting and/or diarrhea and kidney compromise) that you and your veterinarian have noted are due solely to hyperthyroidism or are a part of other disease processes.

Options for Treatment:
Hyperthyroid cats can be treated in many ways including surgery, anti-thyroid medications (pills), and injectable radiation therapy (I-131). Each approach has its pros and cons, but the consensus among veterinarians is that I-131 therapy is the curative treatment of choice for most hyperthyroid cats.

Surgery – may cure a hyperthyroid cat, but recurrence of the problem is common if only one thyroid gland is removed or there is ectopic thyroid tissue (thyroid tissue that grows outside of the thyroid gland). If both thyroid glands are removed, the parathyroid glands may be damaged, requiring several days of hospitalization, monitoring and lifelong supplementation with an active form of Vitamin D. Older cats who develop hyperthyroidism may also be poor anesthetic / surgical candidates.

Medical Management – Methimazole (Tapazole) is the most common drug, and usually controls hyperthyroidism, but does not cure it. Administration of pills or liquids once to twice a day for life is required – a difficult task in some cats. Transdermal formulations of these medications may not be reliable. Potential side effects of methimazole include liver disease, bone marrow suppression, vomiting, diarrhea, poor appetite, skin problems, and immune-mediated disease. Regularly scheduled laboratory work is important to monitor for efficacy of methimazole therapy and to identify any possible adverse effects.

Radiation Therapy – with the injectable isotope Iodine-131 (I-131) cures almost all treated cats. Fewer than 5% of cats remain hyperthyroid post treatment and require a second treatment. Less than 5% of cats become hypothyroid (produce too little thyroid hormone) and require daily thyroid hormone supplementation. I-131 therapy does not require general anesthesia or surgery. The isotope is taken up by the hyperfunctioning thyroid tumor cells wherever they are located within the cat, but most normal thyroid cells, the parathyroid glands and other nearby structures are not affected. Although the injection is simple (single subcutaneous injection), cats will remain radioactive until the isotope decays. The Federal and State laws require us to “hold” or hospitalize a cat until radioactivity levels emitted are too low to be harmful (typically a matter of 3-4 days). Cats are kept in an isolation area here at the Veterinary Specialty Center of Tucson, and cannot be visited, petted or held by their owners until discharge. Because of these isolation requirements, cats with serious non-thyroidal illnesses (heart failure, diabetes, chronic renal failure, etc.) are NOT good candidates for I-131 therapy.