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The ratio of free to protein-bound thyroxine (FTF) in human serum was measured by ultrafiltration of serum to which 131I-thyroxine was added. The tracer was purified by preliminary dialysis in the presence of serum protein. This resulted in a lower estimate of FTF(.026 ±.004% SD) and free thyroxine (2.1 ±O.4 × 1O−3 μg/100 ml) than has been reported previously in undiluted serum. The difference between these values and those reported by others appears to be due to the removal of contaminants during the preliminary dialysis. Ten-fold dilution of serum resulted in no significant change in free thyroxine concentration. This was consistent with mathematical analysis of the dilution effect. Thyroxine binding was markedly temperature sensitive. Free thyroxine concentration increased significantly within the temperature range seen in febrile patients.
Schussler et al. (Wed,) studied this question.