Hyperthyroidism in Cats

FELINE HYPERTHYROIDISM

THE DISEASE: Hyperthyroidism is a disease of older cats in which the thyroid gland produces excessive amount of thyroid hormone. Although the actual cause is still under investigation, most cases are a result of functional benign tumors of the gland. Recent studies point to a household chemical called PolyBrominatedDiethylEther (PBDE) that has been commonly used as a flame retardant on carpet, upholstery, and small electronic appliances as a cause of the thyroid tumors.

SYMPTOMS: Although excessive hormone is produced by a tumor of the thyroid, the actual tumor lump is usually not evident. The primary symptoms are weight loss (often severe) with a ravenous appetite (although the appetite decreases as the cat gets sicker). A rapid heartbeat, nervousness, behavior changes, and vomiting or diarrhea are also seen. The heart muscle consistently shows degeneration, and this heart disease often becomes the most serious health problem to the cat.

DIAGNOSIS: A simple blood test for thyroid hormone is diagnostic for hyperthyroidism, but other tests may be indicated to evaluate the status of the heart and other organs that have been affected by the high thyroid hormone levels.

PROGNOSIS: Hyperthyroidism is very treatable, and cats often live 5 years or more after their successful treatment, even when they are diagnosed in their late teens.

TREATMENT: There are three modes of treatment for this disease:

Antithyroid drugs are very effective at lowering thyroid levels, but some cats don't tolerate the medication well enough to use long-term, and the medication must be used constantly to control the disease. Methimazole (trade name Tapazole) is normally given in tablet form, but a "transdermal gel" can be compounded that absorbs directly thru the cat's skin for ease of medicating.

Radioactive iodine (I131) is a safe and effective way to destroy only the abnormal thyroid tissue. This treatment requires a stay at a special facility that is qualified to use radioactive medications. The hospital stay is usually from 5 to 7 days. Radioactive iodine therapy is normally curative, and cats do not need to continue to take medication. Recurrances are not common.

Surgery offers a permanent cure if the affected thyroid gland can be removed. If only one gland is involved, only the affected gland may be removed. In many cases both glands are enlarged and must be removed. If both glands are removed, the cat must be observed carefully for the first few days for low blood calcium levels, which can occasionally be a complication that requires treatment. Surgery is relatively minor (and usually less expensive than radioactive iodine treatment), but recurrances are more common than with I131 treatment.