Hyperthyroidism

Hyperthyroidism is the most common endocrine disorder in cats, affecting approximately 10% of cats over 10 years of age. The thyroid gland, which sits on either side of the trachea in the neck, produces hormones that regulate metabolism. In hyperthyroid cats, benign growths (adenomas) on one or both thyroid lobes produce hormone continuously without the normal feedback control.

The resulting hormonal excess throws the entire body into metabolic overdrive. The heart beats faster and harder — over time this leads to hypertensive heart disease and cardiac muscle thickening. Blood pressure rises, which can damage the eyes, kidneys, and brain. Despite eating ravenously, cats lose muscle mass and bodyweight because their cells cannot keep pace with the elevated metabolic demand.

Diagnosis is straightforward with a simple blood test measuring total T4 levels. Treatment options include daily oral methimazole (which blocks thyroid hormone synthesis), a prescription iodine-restricted diet (Hill's y/d), radioactive iodine therapy (I-131 — a one-time curative treatment with excellent outcomes), or surgical thyroidectomy.

Radioactive iodine is considered the gold standard — it selectively destroys overactive thyroid tissue while sparing normal tissue, is curative in over 95% of cases, and requires only a short hospital stay. However, it must be performed at a specialist facility. Medical management with methimazole is effective and affordable, though it requires lifelong daily administration and regular monitoring.