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.