The reversible combination of thyroxine with certain of the plasma proteins clearly provides an efficient transport mechanism enabling microgram quantities of the hormone to be carried to tissue sites with minimal losses via glomerular filtration and other nonselective excretory pathways. The demonstration of thyroxine-binding proteins in other extracellular fluids (1) and within cells (2) suggests a means by which thyroxine may be selectively distributed to tissues according to their individual needs.The role, if any, of the plasma thyroxine-binding proteins in various thyroid disorders is not clear. In vitro studies of the protein-thyroxine complex involve alterations of pH, Pco2, temperature and other aspects of the biochemical environment and may not give an accurate index of in vivo relationships. Furthermore, such studies neglect interactions of plasma thyroxine-binding proteins and extravascular thyroxine-binding proteins.It seemed reasonable to suppose that the rate of disappearance of I131-labeled L-thyroxine (T4*) from the intravascular compartment during an acute experiment might yield information about the rate at which equilibrium between intra-and extravascular compartments was approached. The rate of disappearance should reflect the ability of the serum proteins to bind thyroxine and retain it within the vascular compartment in the face of all ambient forces favoring its removal. Conventional studies of labeled thyroxine survival (measured in days) are presumed to measure primarily the rate of thyroxine disposal after equilibration between labeled and stable thyroxine has been attained throughout the body. In preliminary studies, attempts were made to measure the rate

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