What is Feline Hyperthyroidism?
Feline hyperthyroidism is recognized as the most common endocrine disorder in the cat. In 97% of feline hyperthyroid cases, the cause is a benign tumor that over-secretes thyroid hormones, resulting in a multi-systemic disease. The total metabolism 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, and kidney compromise to name a few) 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 if there is ectopic thyroid tissue in the chest cavity. If both thyroid glands are removed, the parathyroid glands may be damaged, requiring several days of hospitalization and/or monitoring to try to avoid a possibly fatal outcome. Older cats who develop hyperthyroidism may also be poor surgical/anesthetic candidates.
Medical Management - Methimazole 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 is often not very effective or 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 I-131 cures almost all treated cats. Fewer than 10% of cats remain hyperthyroid and require a second treatment, while less than 5% become hypothyroid and then require daily thyroid hormone supplementation. I-131 therapy does not require general anesthesia or surgery. The isotope is taken up by the hyperfunctional thyroid cells wherever they are located within the cat, but most normal thyroid cells and the parathyroid gland and other nearby structures are not affected. Although the injection is simple (subcutaneous single injection), cats will remain radioactive until the isotope decays. The Federal and State laws require us to 'hold' a cat until radioactivity levels emitted are too low to be harmful (typically a matter of seven days). Cats are kept in an isolation area here at the Animal Referral and Emergency Center of Arizona, and cannot be visited, petted or held by their owners for the first three days. After this time, they are moved out of isolation and you can visit them between the hours of 9am and 9pm. 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.