Feline hyperthyroidism is primarily caused by benign adenomatous hyperplasia of the thyroid gland, leading to excessive production of thyroxine (T4) and triiodothyronine (T3). The resulting hypermetabolic state presents clinically with weight loss despite polyphagia, tachycardia, polyuria, and polydipsia. Elevated circulating thyroid hormones increase the basal metabolic rate and sympathetic nervous system tone, often resulting in secondary hypertrophic cardiomyopathy. Diagnostic confirmation relies on elevated total or free T4 levels. Current therapeutic modalities include anti-thyroid medications (methimazole), which inhibit thyroid peroxidase; surgical thyroidectomy; and radioactive iodine (I-131) therapy, which selectively destroys hyperfunctioning thyroid tissue and is considered the gold standard for a permanent cure.