Thyroxine, or 3:5,3':5' tetra­iodothyronine (often abbreviated as T4) is the major hormone secreted by the follicular cells of the thyroid gland. T4 is transported in blood, with 99.95% of the secreted T4 being protein bound, principally to thyroxine ­binding globulin (TBG) and to a lesser extent to thyroxine binding pre­albumin (TBPA) and albumin. T4 is involved in controlling the rate of metabolic processes in the body and influencing physical development.