Hyperthyroidism is the most common endocrine disorder of cats, causing progressive weight loss, hyperactivity, and cardiovascular strain. Early diagnosis and treatment markedly improve welfare outcomes.
Hyperthyroidism causes chronic physiological disturbance that significantly impairs affected cats' welfare, though the hyperactive presentation can mislead owners into thinking their older cat has simply become more active. In reality, the metabolic overdrive causes muscle wasting, cardiovascular strain, gastrointestinal disturbance, and hypertension that damages the eyes and kidneys. Without treatment, the condition is progressive and ultimately fatal. The welfare case for treatment is strong as outcomes are generally excellent with appropriate management. Important welfare considerations in treatment choice include the daily burden of medicating a difficult cat, the welfare implications of hospital isolation during radioiodine therapy, and the risk of unmasking underlying kidney disease when hyperthyroidism is corrected. These factors require individualised assessment.