Hyperthyroidism affects around 10% of cats over 10 and is rare in dogs. A benign nodule (occasionally a tumour) in the thyroid gland overproduces thyroid hormone, ramping up the body's metabolism.

Classic signs: weight loss despite voracious appetite, restlessness, hyperactivity, vocalising at night, vomiting, diarrhoea, increased thirst, sometimes aggression or behavioural change. Senior cat owners often describe "they're hungry all the time but losing weight."

Diagnosis is a single blood test (T4). Treatment has four options: - **Methimazole** — oral or transdermal medication, twice daily, lifelong, monitored with regular blood tests - **Iodine-restricted prescription diet** — must be the only food the cat eats - **Radioactive iodine (I-131)** — single treatment, curative, requires brief isolation, available at specialist centres - **Surgery** — uncommon in modern practice

Untreated hyperthyroidism damages the heart and kidneys and substantially shortens lifespan. Treated, most cats live a normal lifespan.