A condition that occurs when the thyroid gland makes more thyroid hormones than the body needs. Thyroid hormones control the way the body uses energy and affect the body’s metabolism. Hyperthyroidism may be caused by an autoimmune disorder called Graves’ disease, benign lumps in the thyroid gland, and inflammation of the thyroid gland. It may also be caused by consuming too much iodine in the diet or by taking too much thyroid hormone medicine. Signs and symptoms include weight loss, fatigue, rapid or irregular heartbeat, sweating, diarrhea, nervousness, mood swings, shaky hands, trouble sleeping, trouble tolerating heat, muscle weakness, and a goiter (an enlarged thyroid gland that may cause the bottom of the neck to look swollen). Hyperthyroidism can usually be controlled with treatment. Also called overactive thyroid.