Hyperthyroidism is the most common endocrine disorder of older cats, causing chronic suffering when untreated but manageable to good quality of life through medication, surgery, or radioiodine treatment.
Cats with untreated hyperthyroidism experience a hypermetabolic state that causes chronic discomfort, muscle wasting, hypertension with associated ocular and neurological complications, and progressive cardiac disease. The condition causes suffering across months to years as it progresses. Medical management with methimazole requires twice-daily dosing which some cats resist. Radioiodine treatment, while requiring a brief hospitalisation period, provides permanent cure and is the highest-welfare long-term outcome. Early diagnosis through routine senior screening prevents advanced disease.