Lack of significant binding of L-triiodothyronine by thyroxine-binding globulin in vivo as demonstrated by acute disappearance of 131-I-labeled triiodothyronine.

Since Gordon and his colleagues' (1) description of the association of thyroxine (T4) with an a-globulin [later termed "thyroxine-binding globulin" (TBG) ] in 1952, thyroid hormone transport has been the subject of intense study. The demonstration of "thyroxine-binding prealbumin" (TBPA) by Ingbar (2) in 1958 provided further understanding of the binding of T4 by the plasma proteins. It has also been shown that the exogenous administration of estrogens (3) or increased endogenous production as in pregnancy (4, 5) is accompanied by an elevation of the thyroxine binding capacity of TBG. Other substances (e.g., androgens) have been found to decrease the binding capacity of TBG (6) or to interfere with it through a competitive action [e.g., diphenylhydantoin (DPH)] (7, 8). Of the previous studies, most have related to thyroxine specifically rather than to triiodothyronine. The available data on the binding of 3,5,3'-L-triiodothyronine (T3) in human plasma are derived from in vitro studies, performed under unphysiological conditions of pH, ionic strength, and temperature, which may alter the binding pattern. Most studies agree, however, that T, is bound mainly by TBG and secondarily by albumin, with