Hyperthyroidism is the most common hormonal disorder in older cats, causing significant suffering if untreated but responding well to several effective therapies.
Untreated hyperthyroidism causes significant chronic suffering. The elevated thyroid hormone creates a state of constant physiological stress: the heart races, metabolism burns through muscle and fat, and cats experience unrelenting hunger even as they waste away. The hyperactivity owners sometimes interpret as their older cat 'acting young again' is actually the manifestation of pathological hormonal excess. Concurrent kidney disease complicates treatment as hyperthyroidism can mask declining kidney function. Despite these complexities, hyperthyroidism is among the most rewarding conditions to treat in senior cats - well-managed cats gain weight, become calmer, and clearly feel better after their thyroid levels normalize.