The Metabolic Significance of Proteinbound Iodine of Plasma: A Study of Its Concentration Under Various Conditions and of Its Rate of Formation as Measured With Radioactive Iodine1

THE FRACTION of plasma iodine designated “protein-bound” is the iodine that is precipitated from plasma along with its proteins by such agents as tungstic acid, zinc hydroxide, or acetic acid in the presence of heat and in addition cannot be freed from these proteins by simple washing. Although the chemical identity of this fraction is as yet poorly understood, its concentration in plasma has gained wide use as an indicator of thyroid activity in man. (Salter et al., 1941; Man et al., 1942; Lowenstein et al., 1944; Talbot et al., 1944; Chaney, unpublished observations). In the present investigation, an attempt has been made to determine the metabolic significance of plasma protein-bound iodine. Its concentration in plasma was studied in rats shown to be completely deprived of thyroid tissue and in rats and guinea pigs in which hyperthyroidism had been induced. The use of radioactive iodine (I131) as a labeling agent also made possible a study of the rate of formation of protein-bound iodine in both states of thyroid activity.