Of all the chronic diseases that plague older cats, hyperthyroidism is among the most common. Symptoms can include increased appetite, weight loss, excessive drinking and urination, increased vocalisation or howling, vomiting, and hyperactivity. Thyroid medication can control the disease and limit the damage it does to the cat's heart and kidneys.
Methimazole (brand names Tapazole and Felimazole) is the drug used most commonly for hyperthyroid cats. Carbimazole is another form of methimazole. Although it is not sold commercially in the U.S., it is available from compounding pharmacies and causes fewer side effects than Tapazole and Felimazole do.
Side effects of Tapazole and Felimazole in cats can include scratching around the ears, lethargy, loss of appetite, and vomiting. More serious side effects are liver toxicity and changes in the bone marrow. Starting the medication at a low dose and working up to a therapeutic level can reduce the possibility of side effects.
Thyroid medication must be given twice a day for the life of the cat. Regular blood tests are necessary to determine whether the dose is correct.
Alternatives to Pills
Thyroid medication for cats can be compounded into fish- or chicken-flavoured liquid, soft moist treats, or a gel to rub inside an ear tip.
A single injection of radioactive iodine can cure a cat's hyperthyroidism. Your veterinarian can tell you about nearby treatment facilities.