Feline hyperthyroidism is the most common endocrine disorder in middle-aged and older cats. Radioiodine (I-131) therapy offers the closest thing to a permanent cure and is widely considered the gold-standard treatment.
Hyperthyroidism causes chronic welfare compromise that owners often misattribute to normal aging. An affected cat is constantly in a hypermetabolic state — essentially running an internal engine at too-high speed — experiencing increased heart rate, anxiety, muscle breakdown, and often gastrointestinal distress. Many owners describe their cat as a "different animal" after successful treatment, returning to their pre-illness personality. The isolation period required after radioiodine therapy is a welfare concern, particularly for cats who find the specialist facility stressful, but this short-term cost is vastly outweighed by the long-term benefit of resolution. Welfare advocacy involves educating cat owners about hyperthyroid signs so diagnosis and treatment happen earlier.