Feline hyperthyroidism is the most common endocrine disorder in older cats, causing weight loss, hyperactivity, and serious secondary organ damage if untreated. Multiple effective treatment options allow most cats to achieve good quality of life with appropriate management.
Uncontrolled hyperthyroidism causes chronic discomfort through weight loss, gastrointestinal disturbance, anxiety from hyperactivity, and cardiovascular strain. Many cats are diagnosed late because weight loss and behaviour change are attributed to normal ageing. Early diagnosis through annual thyroid screening for cats over seven years enables treatment before secondary organ damage occurs. Medical management with twice-daily methimazole requires compliance that can be challenging; long-acting transdermal formulations applied to the ear improve compliance. All treatment options carry welfare implications — radioactive iodine requires weeks of isolation post-treatment. Choosing the treatment that matches the individual cat's circumstances and the owner's capacity for ongoing management is key to long-term welfare success.