The thyroid gland is located in the front of the neck below the voice box. It is just about the same size and shape – and is in the same location – as a small bow tie. The thyroid secretes two hormones that are crucial for regulating metabolism: triiodothyronine (T3) and thyroxine (T4). The numbers refer to the numbers of iodine atoms each molecule of hormone contains. T4 is the major player, because it affects virtually every cell in the body.