Feline hyperthyroidism is the most common endocrine disorder in older cats, causing significant physical deterioration from chronic thyroid hormone excess that responds well to several available treatment approaches.
Hyperthyroid cats experience a state of chronic metabolic overactivation that progressively undermines their health. Despite an apparently good appetite, affected cats lose weight as their metabolism outpaces calorie intake. The cardiovascular system bears a particularly heavy burden — heart rate increases, cardiac muscle thickens, and blood pressure rises. Untreated, hypertrophic cardiomyopathy from hyperthyroidism leads to heart failure. Hypertension causes retinal damage and potential blindness. The restlessness and anxiety that hyperthyroid cats show likely reflects genuine physiological distress from sympathetic nervous system overactivation. Early diagnosis and effective treatment reverse most of these welfare impacts, restoring normal body weight, cardiac function, and behavioural calm in most cats.