Feline hyperthyroidism — overproduction of thyroid hormones from a benign thyroid tumor — is the most common endocrine disorder in older cats, causing weight loss, behavioral changes, cardiovascular disease, and significant suffering if untreated.
Hyperthyroidism causes real suffering that is sometimes mistaken for normal aging. A cat losing weight, vocalizing at night, showing increased anxiety, and developing heart problems is not "just getting old" — it is experiencing the physiological consequences of thyroid hormone excess. The elevated heart rate alone is stressful and over time causes cardiac hypertrophy that impairs heart function. Untreated hyperthyroid cats also frequently develop hypertension that damages the eyes, kidneys, and brain. The welfare imperative is prompt diagnosis when clinical signs appear and consistent, appropriate treatment. Radioactive iodine is curative but requires a period of isolation at a specialist facility — well worth the short-term stress for long-term welfare benefit.