THE EFFECT OF ESTROGENS UPON THE PERIPHERAL METABOLISM OF THYROXINE *†

Abstract

The effects of diethylstilbestrol and of estradiol benzoate upon the kinetics of thyroxine metabolism have been assessed in patients with intact thyroidal function, treated myxedema, or Graves'' disease. In all subjects, estrogen induced an increase in the binding avidity of the thyroxine-binding globulin (TBG), which was accompanied by marked retardation of the fractional rate of peripheral turnover of I13l-labeled thyroxine. In eumetabolic subjects, concentration of protein-bound iodine (PBI) increased sufficiently to restore the daily disposal of hormonal I to normal values. Two of four thyrotoxic patients failed to demonstrate expected increases in the concentration of PBI, and exhibited amelioration of symptoms and of hypermetabolism during administration of estrogen. During this period, daily hormonal disposal rates approached values found subsequently during restoration of eumetabolism by propylthiouracil. These observations are consistent with the hypothesis that thyroxine-binding by TBG exerts a rate-limiting effect upon the peripheral metabolism of this hormone.