Hyperthyroidism is the most common endocrine disorder of older cats, causing significant health and welfare impacts if undiagnosed or inadequately managed.
Untreated hyperthyroidism causes progressive deterioration: chronically elevated metabolic rate causes muscle wasting, cardiac changes including hypertrophic cardiomyopathy, hypertension, and ultimately organ failure. Cats may appear hyperactive initially but become progressively debilitated. Owners often misinterpret early weight loss as benign ageing. Effective treatment dramatically transforms welfare outcomes, with most cats returning to normal condition and behaviour within weeks. Importantly, treatment must be introduced carefully to avoid masking concurrent renal disease, as normalising thyroid function can unmask pre-existing kidney disease. Regular screening of senior cats for hyperthyroidism is one of the most impactful welfare interventions available.