A Study of the Nature of the Circulating Thyroid

The physico-chemical form in which the secretion of the thyroid gland exists in blood is not well understood. It is generally believed to be associated in some manner with the plasma proteins since the iodine content of precipitated washed plasma protein (1) and the rate of turnover of 131 in this protein appears to vary with the state of thyroid activity (2). The observation of Trevorrow (3) that this "protein bound" or "hormonal iodine" is readily extractible with butanol from unhydrolysed serum indicates that it is not bound by a peptide linkage. The introduction of the technique of paper electrophoresis combined with the use of I131 as a tracer isotope has afforded a new and valuable instrument for a study of the problem (4, 5). With this technique it has been demonstrated that after 24 to 48 hours there is a characteristic localization of administered I131 in the alpha globu-