Kristofer S. Fritz, Rene M. Weiss, Jerald C. Nelson, R. Bruce Wilcox. "Unequal Concentrations of Free T3 and Free T4 after Ultrafiltration." Clinical Chemistry 53.7 (2007): 1384-1385. ( 7/2007 )
Ultrafiltration is a standard method for separating free T3 and free T4 from serum proteins and protein bound hormone (1, 2). It is expected that free T3 and free T4 in aqueous solutions will accompany water as water moves across semipermeable membranes, a process that results in equal concentrations of free T3 and free T4 in the aqueous solutions on opposite sides of semipermeable membranes after ultrafiltration. The movements of free thyroid hormones through semipermeable membranes
have not been compared to the movement of water, nor have the concentrations of free hormones on opposite sides been determined in the absence of hormone-binding proteins. We report experiments with 4 different ultrafiltration devices to determine
the movements of 3H2O, 125IT3, and 125I-T4 across semipermeable membranes in a biologically relevant protein-free aqueous solution.
A recent study by the authors documented inconsistency in the retention of serum proteins by these same devices. The present study confirms the findings of the previous study; ultrafiltration is complex, poorly characterized, and incompletely understood. The movement of water, T3, and T4 across semipermeable membranes could not be predicted based on the reported MWCO. These inconsistencies complicate the interpretation of previous free thyroid hormone measurements involving uncharacterized ultrafiltration.