Hyperthyroidism is the most common endocrine disease of older cats, affecting approximately 10% of cats over 10 years old. It causes rapid weight loss, hypertension, and cardiac problems if untreated, but responds well to treatment.
Hyperthyroid cats experience a chronic state of physiological overdrive: rapid heart rate, increased blood pressure, weight loss despite hunger, and anxiety-like hyperactivity. Untreated hypertension causes blindness, heart disease, and kidney damage. Cats that are treated effectively show dramatic welfare improvement as clinical signs resolve. Radioiodine therapy provides a permanent cure but the hospitalisation period causes welfare concern for cats that find confinement stressful.