The Effects of Noncalorigenic Congeners of Salicylate on the Peripheral Metabolism of Thyroxine.

Various experiments have indicated the presence in whole human serum of three major thyroxine (T4)-binding 1 proteins: T4-binding globulin (TBG), T4-binding prealbumin (TBPA), and albumin (1, 2). It is well established that, in vitro, TBG and TBPA are the major T4-binding proteins, whereas albumin is a relatively weak secondary carrier (3-5). These proteins interact with the hormone in a reversible binding equilibrium in which the majority of the hormone is bound but a measurable proportion is unbound or free. Free T4 has been suggested as the metabolically active component available to the cells in vivo for degradation, excretion, and initiation of hormonal action, with the bound hormone acting as a metabolically inert reservoir (3-5). The role of TBG in the transport and peripheral metabolism of T4 in man is well accepted. This acceptance is based on studies demonstrating a consistent relationship between changes in the T4-binding activity of TBG, as assessed by electrophoresis in vitro and changes in the concentration and fractional turnover of T4 in vivo (6-8). From these observations, the conclusion has been drawn that TBG regulates the peripheral metabolism of T4 by limiting the concentration of unbound hormone. The existence of TBPA as a normal constituent of human serum has been satisfactorily demonstrated by several techniques, and earlier doubts concerning its function in binding T4 at physiological pH have been dispelled (5, 9, 10). The role, if any, of -TBPA in regulating the peripheral metabolism of T4 in vivo has remained uncertain.