Hyperthyroidism is the most common endocrine disorder in older cats, causing significant metabolic welfare harms that are largely reversible with appropriate treatment.
Uncontrolled hyperthyroidism causes chronic physiological stress — elevated heart rate, muscle wasting, and gastrointestinal disturbance over months or years. Concurrent hypertension causes retinal haemorrhage and blindness in severe cases. Cats with uncontrolled hyperthyroidism are chronically uncomfortable despite appearing active. Post-treatment, cats often show marked improvement in demeanour and weight — owners frequently report that their cat seems 'years younger'. Untreated cats face progressive cardiac damage (hypertrophic cardiomyopathy) and renal complications.