Feline hyperthyroidism has become one of the most common disorders in older cats since its recognition in the 1970s. Effective treatments exist but require careful long-term management.
Hyperthyroid cats experience significant welfare deterioration before diagnosis. Uncontrolled thyroid hormone causes cardiovascular stress, hypertension with associated retinal detachment and blindness, weight loss despite hyperphagia, and generalised hyperexcitability that may be distressing. The cardiovascular sequelae — hypertrophic cardiomyopathy and hypertension — cause cumulative organ damage. Effective treatment normalises quality of life dramatically. Radioactive iodine (I-131) is considered the gold standard treatment — a single injection provides permanent cure in over 95% of cats. Medical management with methimazole requires twice-daily medication and regular monitoring. Dietary management using iodine-restricted food is less reliable but avoids medication. A key welfare consideration is unmasking pre-existing kidney disease during treatment — normalising thyroid function can reveal compromised renal function requiring careful management.