Hyperthyroidism is the most common endocrine disorder in older cats, causing multi-system effects including serious cardiac disease that significantly affect welfare when untreated.
Untreated hyperthyroidism causes sustained physiological stress comparable to living in a permanently anxious, hyperactive state. Affected cats lose weight despite ravenous appetite, experience cardiovascular strain, and live with the discomfort of gastrointestinal upset and hypertension. Secondary hypertensive retinopathy can cause sudden irreversible blindness - a welfare catastrophe that is entirely preventable with treatment. Cardiac hypertrophy from untreated hyperthyroidism can progress to congestive heart failure. The excellent response to treatment - particularly radioactive iodine which is curative - makes hyperthyroidism one of the most rewarding conditions to diagnose and treat from a welfare perspective, as treatment so dramatically restores normal health and comfort.