Feline hyperthyroidism is the most common endocrine disease in older cats. Treatment options each have distinct welfare profiles and should be matched to individual cat needs.
Hyperthyroidism causes significant welfare harm through its systemic effects. Despite paradoxically increased appetite, affected cats lose weight and muscle mass as their metabolism is driven above sustainable levels. Heart rate increases, causing hypertensive cardiomyopathy and hypertension that can damage eyes, kidneys, and brain. The persistent internal urgency of excess thyroid hormone causes behavioral changes — restlessness, yowling, aggression — that owners often attribute to aging rather than disease.
Prompt diagnosis and treatment is welfare-critical. Blood pressure measurement at diagnosis is essential — uncontrolled hypertension causes retinal detachment and sudden blindness, an avoidable welfare harm. Anti-hypertensive treatment (amlodipine) should be initiated immediately if hypertension is confirmed.
Methimazole/carbimazole medication requires twice-daily administration for life and can cause side effects including bone marrow suppression. Radioactive iodine requires a short period of quarantine hospitalization but is curative, eliminating lifelong medication burden. Dietary management requires strict adherence — any alternative food source disrupts treatment. Welfare-centered treatment selection considers the individual cat's temperament, the owner's circumstances, and concurrent disease status.