Measurement of Free Thyroxine Concentration in Human Serum*

In the present investigation the existence of "free" thyroxine in human serum was repeatedly verified by dialysis through cellophane, with paper chromatographic identification of thyroxine in the dialysate.Methods were developed for measurement of the minute amount of thyroxine present in the dialysate that was considered to represent the unbound or diffusible fraction of the hormone in serum.Similar values were obtained whether a tracer amount of labeled thyroxine was added to serum, or serum containing endogenously labeled hormone after I131 therapy was employed. MATERIALS AND METHODS MaterialsI"1-labeled L-thyroxine was obtained from E. R. Squibb and Co. or from Abbott Laboratories.Shipments were tested for purity by descending paper chromatog- raphy as previously described (1, 2), and were discarded if found to be less than 95 per cent pure.Nonradioactive L-thyroxine was purchased from Mann Research Co., New York City.All blood sera not used the day they were drawn were stored frozen.Repeated freezing and thawing had no effect upon the results.Large pools of sera were sup- plied by the Medical Chemistry Laboratory and run as standards with each set of dialysis experiments. Clinical materialThyrotoxicosis.Sixty hyperthyroid patients were available from the Thyroid Clinic and the Department of Ra- diological Research of the Presbyterian Hospital.Blood samples were obtained from 35 patients 1 to 13 days after 1131 therapy.The remaining 25 patients did not receive radioactive iodine therapy before study of their blood sera.The diagnoses were verified by thyroidal uptake of I"1, serum protein-bound iodine (PBI) determination