Analysis of the Rapid Interchange of Thyroxine Between Plasma and Liver and Plasma and Kidney in the Intact Rat

Abstract

Tissue uptake of intravenously administered 125I-thyroxine (125I-T4) was estimated in liver and kidney after appropriate corrections for tracer T4 associated with plasma proteins in these organs. By simultaneous measurements of 125I-T4 in liver and kidney the kinetics of interchange of T4 between liver and plasma and kidney and plasma could be analyzed in terms of a nonrestricted 3-compartmental system. The unidirectional hepatic clearance of plasma T4 was estimated to be 1.7 ml/min. and the unidirectional clearance of plasma T4 by the kidney 0.18 ml/min. The rapid 2-way exchange of T4 between plasma and tissues was not accompanied by the formation of demonstrable metabolic intermediates. Equilibration between plasma and tissue was complete after 10 to 15 min. The exchangeability of tissue T4 was experimentally demonstrated by exchange transfusions. Electrophoretic analysis of tissue homo-genates indicated that the ingress of T4 from plasma could not be attributed to high tissue concentrations of soluble plasma thyroxine-binding proteins. Cellular factors are responsible for the reversible accumulation of thyroxine both in liver and kidney.