The thyroid is a butterfly-shaped gland that is located in the front of the neck just above the trachea; it weighs approximately 15 g to 20 g in the adult human. The thyroid produces and releases into the circulation at least two potent hormones, thyroxine (T4) and triiodothyronine (T3), that influence basal metabolic processes or enhance oxygen consumption in nearly all body tissues. Thyroid hormones also influence linear growth; brain function, including intelligence and memory; neural development; dentition; and bone development.