Hyperthyroidism is the most common endocrine disease in older cats and, if untreated, causes progressive deterioration and suffering. Multiple effective treatment options allow most cats to achieve sustained remission with good quality of life.
Untreated hyperthyroidism creates a distinctive welfare profile: cats appear energetic and have good appetites but are experiencing significant systemic stress. The hyper-metabolic state consumes muscle mass, elevates heart rate chronically, and generates secondary hypertension that can cause acute blindness from retinal detachment. The ravenous appetite with simultaneous weight loss is physiologically distressing — cats are hungry and eating but cannot maintain body condition. Cardiac hypertrophy from chronic thyroid excess predisposes to life-threatening arrhythmias and congestive heart failure. Most owners recognise the urgency of treatment once informed of these consequences. The treatment welfare burden varies by method: daily oral medication requires owner compliance and causes intermittent cat stress from handling; transdermal gel is easier to administer; radioactive iodine involves a period of hospitalisation but produces durable cure. Monitoring for the emergence of CKD after treatment normalises thyroid function is a critical welfare consideration that prevents over-correcting one condition while unmasking another.