Hyperthyroidism is characterized by overproduction of thyroid hormone in the thyroid gland and affects 1.3% of adults in the United States. Thyrotoxicosis is a state of thyroid hormone excess and may be caused by hyperthyroidism, thyroiditis, or exogenous administration. The most common symptoms of hyperthyroidism are weakness, palpitations, weight loss, and heat intolerance, and the most common signs are a palpable goiter, tachycardia, muscle weakness, and tremor. A low thyroid-stimulating hormone (thyrotropin) level has a high sensitivity and specificity for diagnosing thyrotoxicosis. The most common cause of hyperthyroidism is the autoimmune condition Graves disease, typically diagnosed by the presence of thyroid eye disease, which is pathognomonic, or thyrotropin receptor antibodies. Other causes of hyperthyroidism are toxic multinodular goiter, toxic adenoma, and thyroiditis, which can be differentiated by the pattern of uptake on a radioactive iodine scan. Thionamides (most commonly methimazole) typically induce remission of Graves disease and can control hyperthyroidism caused by multinodular goiter and toxic adenoma. Radioactive iodine resolves hyperthyroidism in more than 90% of patients with Graves disease and toxic multinodular goiter, with hypothyroidism developing in most patients 1 year after treatment. Thyroidectomy is the treatment of choice for patients with compressive symptoms from an obstructive goiter.