Hyperthyroidism is the most common endocrine disease of older cats, causing significant chronic welfare impact that is highly treatable with several effective options including radioiodine.
An untreated hyperthyroid cat is living with constant physiological overdrive - elevated heart rate, hypertension, weight loss despite excessive eating, and the cumulative effects of untreated hypertension on kidneys, heart, and eyes. This is not a benign aging change but a treatable disease causing measurable suffering. The welfare-optimal treatment for most cats is radioiodine therapy: a single treatment achieves cure in over 95% of cases, eliminating the daily stress of medication and providing the best long-term health outcomes. The treatment requires a short period of isolation in a licensed facility which is a minor welfare trade-off against years of improved health. Early diagnosis through annual blood testing in senior cats allows treatment before secondary damage occurs.