Hyperthyroidism is a pathological syndrome in which tissue is exposed to excessive amounts of circulating thyroid hormone. The most common cause of this syndrome is Graves' disease, followed by toxic multinodular goitre, and solitary hyperfunctioning nodules. Autoimmune postpartum and subacute thyroiditis, tumours that secrete thyrotropin, and drug-induced thyroid dysfunction, are also important causes. The diagnosis of hyperthyroidism is generally straightforward, with raised serum thyroid hormones and suppressed serum thyrotropin in almost all cases. Appropriate treatment of hyperthyroidism relies on identification of the underlying cause. Antithyroid drugs, radioactive iodine, and surgery are the traditional treatments for the three common forms of hyperthyroidism. Beta-adrenergic blocking agents are used in most patients for symptomatic relief, and might be the only treatment needed for thyroiditis, which is transient. The more unusual causes of hyperthyroidism, including struma ovarii, thyrotropin-secreting tumours, choriocarcinoma, and amiodarone-induced thyrotoxicosis are, more often than not, a challenge to diagnose and treat.