Feline hyperthyroidism is the most common hormonal disorder in older cats — highly treatable, yet often causes prolonged suffering when undiagnosed due to its gradual, insidious onset.
The insidious welfare cost of hyperthyroidism is that cats appear energetic and hungry — which owners often read as health — while their hearts, kidneys, and blood pressure are being chronically damaged. By the time lethargy and poor coat appear, disease is advanced. Weight loss in an older cat eating voraciously is a classic red flag that warrants thyroid testing. Once diagnosed, treatment dramatically and rapidly improves welfare: most cats become calmer, regain body condition, and show improved coat quality within weeks of treatment. The choice of treatment method should balance efficacy, practicality, and concurrent disease — cats with CKD require particularly careful titration of methimazole to avoid precipitating renal crisis. Annual blood screening for cats over 8 catches hyperthyroidism before cardiac and hypertensive complications develop.