Hyperthyroidism results when the thyroid gland, located in the front of the neck, produces too much thyroid hormone. The thyroid controls metabolism, so if it is overactive, it may lead to weight loss, hair loss, a fast heartbeat, sweating, tiredness, shakiness, and moodiness. Graves' disease is the most common type of hyperthyroidism. Hyperthyroidism is easily treated, usually with radioactive iodine and antithyroid medicine. Follow the links below to find WebMD's comprehensive coverage about hyperthyroidism and how it develops, how it is diagnosed and treated, and much more.