Hyperthyroidism, often called overactive thyroid and sometimes hyperthyreosis, is a condition in which the thyroid gland produces and secretes excessive amounts of the free (not protein bound circulating in the blood) thyroid hormones -triiodothyronine (T3) and/or thyroxine (T4). Graves’ disease is the most common cause of hyperthyroidism. The opposite is hypothyroidism (‘sluggish thyroid’), which is the reduced production and secretion of T3 and/or T4. Hyperthyroidism is one cause of thyrotoxicosis—the hypermetabolic clinical syndrome which occurs when there are elevated serum levels of T3 and/or T4. Thyrotoxicosis can also occur without hyperthyroidism. Some people develop thyrotoxicosis due to inflammation of the thyroid gland (thyroiditis), which can lead to excessive release of thyroid hormone already stored in the gland (without the accelerated hormone production that characterizes hyperthyroidism). Thyrotoxicosis can also occur after taking too much thyroid hormone in the form of supplements, such as levothyroxine (a phenomenon known as exogenous thyrotoxicosis, alimentary thyrotoxicosis, or occult factitial thyrotoxicosis). Thyroid imaging and radiotracer thyroid uptake measurements, combined with serologic data, enable specific diagnosis and appropriate treatment. Management differ for thyrotoxicosis caused by hyperthyroidism and thyrotoxicosis caused by other conditions. This often includes: initial temporary use of suppressive thyrostatics medication (antithyroid drugs), and possibly later use of permanent surgical or radioisotope therapy. All approaches may cause underactive thyroid function (hypothyroidism) which is easily managed with levothyroxine or triiodothyronine supplementation. Surgery as an option predates the use of the less invasive radioisotope therapy (radioiodine 131 thyroid ablation), but is still required in cases where the thyroid gland is enlarged and causing compression to the neck structures, or the underlying cause of the hyperthyroidism may be cancerous in origin.