The transport into rat liver of thyroxine (T4) bound to human prealbumin was studied with the use of sera obtained from patients with thyroid hormone-binding globulin (TBG) deficiency and with purified human prealbumin. The unidirectional extraction of 125I-T4 by liver was measured after rapid injection of isotope mixed in human serum into the portal vein of ketamine-anesthetized rats. The percentage of total serum T4 transported into liver was 47.6 +/- 5.2% in subjects with TBG deficiency, and this represented a 50% increase in hepatic T4 transport relative to control human serum. Since T4 is bound to albumin and to prealbumin in complete TBG deficiency, these results suggested that T4 bound to human prealbumin was transported into rat liver. This was confirmed using portal vein injections of human prealbumin at physiological concentrations (0.1-0.3 mg/ml). At these concentrations, T4 bound to human prealbumin was readily transported into liver. These studies suggest factors present in the liver microcirculation inhibit the binding of T4 to human prealbumin such that T4 bound to human prealbumin is highly transportable in liver; conversely, T4 bound to rat prealbumin is not transportable in rat liver. The inability of human prealbumin to sequester T4 in plasma may provide the basis for the selective advantage in humans of TBG, which does sequester T4 in plasma.