Hyperthyroidism is the most common hormonal disorder in older cats, causing significant welfare harm if untreated — but is very manageable with appropriate treatment.
Hyperthyroidism causes progressive, multi-system welfare harm in older cats. The cardiovascular effects — hypertension and hypertrophic cardiomyopathy — can be life-threatening. However, this condition is very manageable when detected early and treated appropriately. Daily medication (methimazole) is effective but requires owner compliance; radioiodine therapy is curative in a single treatment. The welfare gap between diagnosed and undiagnosed hyperthyroid cats is enormous — treated cats often regain weight, become calmer, and live comfortably for years; untreated cats decline progressively. Annual blood screening for cats over 10 is one of the highest-impact welfare investments available to cat owners.