Quantifying spurious free T4 results attributable to thyroxine-binding proteins in serum dialysates and ultrafiltrates.

BACKGROUND Direct equilibrium dialysis and direct ultrafiltration free thyroxine (T(4)) assays rely on semipermeable membranes to exclude T(4)-binding serum proteins from dialysates and ultrafiltrates. The presence of these proteins in dialysates or ultrafiltrates will yield spuriously high free T(4) values when free T(4) is quantified by RIA. METHODS We used a nonanalog free T(4) RIA that detects and quantifies dialyzable and ultrafilterable serum free T(4) to detect T(4)-binding serum proteins. Two equilibrium dialysis devices and 3 ultrafiltration devices were used to illustrate this application. Displacements of [(125)I]T(4) from anti-T(4) by various concentrations of T(4)-depleted thyroxine-binding globulin, albumin, and serum total protein were compared to displacements by various concentrations of free T(4). RESULTS Both dialysis devices excluded detectable T(4)-binding serum proteins from dialysates. Two of 3 ultrafiltration devices excluded detectable T(4)-binding serum proteins from ultrafiltrates. One did not, and its ultrafiltrate yielded spurious free T(4) values that correlated directly with serum protein concentrations. CONCLUSION The presence or absence of T(4)-binding proteins in dialysates and ultrafiltrates and the spurious free T(4) values that these proteins cause can be documented using a nonanalog free T(4) RIA.