Thyroxine uptake by perfused rat liver. No evidence for facilitation by five different thyroxine-binding proteins

Rates of hepatic uptake of thyroxine (T4) from dilute solutions of five different plasma T4-binding proteins were measured in the isolated perfused rat liver using an indicator dilution method. For each protein, this rate was compared with the rate of spontaneous dissociation of the T4-protein complex measured in vitro. Proteins studied were human T4-binding globulin (TBG), human T4-binding prealbumin (TBPA), human albumin, rat TBPA, and human albumin isolated from subjects with familial dysalbuminemic hyperthyroxinemia. For each of the five protein-hormone complexes studied, the rate of hepatic uptake of T4 (measured under conditions expected to result in dissociation-limited uptake) closely approximated the rate of spontaneous dissociation of the protein-hormone complex within the hepatic sinusoids. These findings indicate an absence of special cellular mechanisms that facilitate the hepatic uptake of T4 from its plasma binding proteins, and support the view that uptake occurs from the free T4 pool after spontaneous dissociation of T4 from its binding proteins.