The effect of estrogen upon the peripheral metabolism of thyroxine

In plasma, thyroxine is avidly bound to specific proteins present in low concentration, particularly the so-called thyroxine-binding globulin (TBG) and thyroxine-binding pre-albumin (TBPA) (1-4). Several observations indicate that such protein-hormone interactions are of physiological importance. In vitro, the ability of cellular sys-tems to accumulate thyroxine from suspending media is conditioned by the thyroxine-binding potency of the proteins within the media (5, 6). In addition, the volume of distribution and rate of turnover of thyroxine in vivo resemble those of certain plasma proteins (7, 8)'. Finally, abnormal thyroxine-binding by TBG and TBPA has been observed in a variety of states in which the periph-eral metabolism of thyroid hormone is disturbed (9-12). It therefore seemed possible that inter-actions with plasma proteins might in part regu-late the peripheral utilization and degradation of thyroxine. An ideal test of this hypothesis would be pro-vided by studies of hormonal metabolism per-formed before and during the administration of large quantities of purified binding protein. How-ever, adequate quantities of these materials are not yet available. Therefore, it. has been necessary to assess the effects of alterations in thyroxine-binding induced by pharmacological means. Larg