Hyperthyroidism is the most common endocrine disorder of older cats, causing weight loss, restlessness, and organ damage that is highly treatable when diagnosed and managed appropriately.
Hyperthyroidism causes chronic suffering in affected cats through multiple mechanisms. Elevated thyroid hormone creates a state of metabolic overdrive — affected cats are often described as agitated, restless, and unable to rest comfortably, equivalent to a persistent anxiety state. Progressive weight loss despite excessive appetite reflects the metabolic demand outpacing nutritional intake. Cardiac complications including hypertrophic cardiomyopathy and hypertension develop with prolonged disease, causing further welfare harm. Gastrointestinal signs including vomiting and diarrhoea add to the burden of disease. The good news is that hyperthyroidism is highly treatable — cats that receive appropriate therapy typically show rapid improvement in demeanour, weight, and quality of life. Medical management with methimazole is effective but requires lifelong daily administration, which some cats find aversive. Radioactive iodine provides permanent resolution in most cases with a single treatment. The welfare priority is early diagnosis through screening of older cats and prompt initiation of treatment, since the disease is progressive and organ damage accumulates with delay.