Thyroxine, or 3:5,3':5' tetraiodothyronine (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 thyroxinebinding globulin (TBG), to a lesser extent thyroxine binding prealbumin (TBPA) and albumin. T4 is involved in controlling the rate of metabolic processes in the body and influencing physical development.
Note: Thyroxine is supposedly a prohormone and a reservoir for the most active and main thyroid hormone T3, T4 being converted as required in the tissues by deiodinases.