Several treatment options for hyperthyroidism exist, each with advantages and disadvantages.
Oral administration of antithyroid medication. Methimazole (brand name TapazoleTM) has long been the mainstay of drug therapy for feline hyperthyroidism. It is highly effective in correcting the condition, often within two to three weeks. Unfortunately, about 10%-15% of cats will suffer side effects, such as loss of appetite, vomiting, lethargy, and occasionally blood cell abnormalities. Rare but more serious side effects include severe facial itching with self-induced trauma, blood clotting disorders, or liver problems. Most side effects are mild and eventually resolve, although some necessitate discontinuation of the medication. Lifelong daily medication is required, which is a disadvantage to owners whose cats resist pilling. CBC and T4 levels need to be rechecked regularly for the remainder of the cat’s life.
Surgical removal of the thyroid gland. Hyperthyroidism is usually caused by a benign tumorcalled a thyroid adenoma that involves one or, more often, both thyroid glands. Fortunately, most hyperthyroid cats have benign, well-encapsulated tumors that are easily removed. Surgery usually results in a palliation and not a cure, but anesthesia can be challenging in these older patients whose disease may have affected their hearts and other organs. Although surgery may seem costly, it often ends up being less expensive than years of oral medication and regular bloodwork rechecks.
Radioactive iodine therapy. This is probably the safest and most effective treatment option. Radioactive iodine, given by injection, becomes concentrated in the thyroid gland, where it irradiates and destroys the hyperfunctioning tissue. No anesthesia or surgery is required, and only one treatment is usually needed to achieve a cure. It used to be that radioiodine treatment was performed only in specialized, licensed facilities, but many private treatment facilities are now found throughout the country. Hospitalization may be prolonged; depending on local or state ordinances, cats may need to be kept at the treatment facility for 10 to 14 days until the level of radioactivity in their urine and feces decreases to an acceptable level. Also, radioiodine therapy is costly. The price tag has come down from about $1,200 to between $500 and $800-but this is still prohibitive for many cat owners.
This information was adapted from an article written by Arnold Plotnick, DVM.