Thyrotoxicosis is a clinical syndrome caused by circulation of excess thyroid hormones. Classic hyperkinetic thyrotoxicosis is readily recognizable. Atypical presentations, however, can lead to diagnostic dilemmas. The sensitive thyroid-stimulating hormone assay has become an invaluable tool in the diagnosis of thyrotoxicosis. Causes of thyrotoxicosis include Graves' disease, toxic multinodular goiter, toxic adenoma, thyroiditis, inappropriate secretion of thyroid-stimulating hormone, trophoblastic tumor, exposure to iodine, and use of drugs. Increased uptake of radioidine by the thyroid gland differentiates true hyperthyroidism from other causes of thyrotoxicosis. Graves' disease is the most common cause of hyperthyroidism. It is caused by antibodies to the thyroid-stimulating hormone receptor that are stimulatory in nature. Extrathyroidal manifestations of Graves' disease include orbitopathy, dermopathy, and acropachy. Therapy includes thionamides, radioactive iodine, surgical intervention, and other adjunctive modalities. Thyroid storm is a medical emergency that necessitates prompt and aggressive therapy.