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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 a prohormone and a reservoir for the active thyroid hormone T3, T4 being converted as required in the tissues by deiodinases.