The thyroid gland releases triiodothyronine (T3) and thyroxine (T4). These hormones play an important role in regulation of your weight, energy levels, internal temperature, skin, hair, nail growth, and more. Thyroxine (T4) is produced by the thyroid gland under regulation from the hypothalamus and pituitary gland. T3 is a second thyroid hormone that is produced by the thyroid gland but also in other tissues through deiodination of T4. T3 helps maintain muscle control, brain function and development, and heart and digestive functions.