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Accepted on ,

Technical Briefs

Quantifying Spurious Free T₄ Results Attributable to Thyroxine-Binding Proteins in Serum Dialysates and Ultrafiltrates,

¹ Department of Biochemistry, Loma Linda University School of Medicine, Loma Linda, CA, USA
² Department of Internal Medicine, Loma Linda, CA, USA

^* To whom correspondence should be addressed. E-mail: jcnelson{at}llu.edu.

Background: Direct equilibrium dialysis and direct ultrafiltration-free thyroxine (T₄) assays rely on semipermeable membranes to exclude T₄-binding serum proteins from dialysates and ultrafiltrates. The presence of these proteins in dialysates or ultrafiltrates will yield spuriously high free T₄ values when freeT₄ is quantified by RIA.

Methods: We used a nonanalog free T₄ RIA that detects and quantifies dialyzable and ultrafilterable serum free T₄ to detect T₄-binding serum proteins. Two equilibrium dialysis devices and 3 ultrafiltration devices were used to illustrate this application. Displacements of [¹²⁵I]T₄ from anti-T₄ by various concentrations of T₄-depleted thyroxine-binding globulin, albumin, and serum total protein were compared to displacements by various concentrations of free T₄.

Results: Both dialysis devices excluded detectable T₄-binding serum proteins from dialysates. Two of 3 ultrafiltration devices excluded detectable T₄-binding serum proteins from ultrafiltrates. One did not, and its ultrafiltrate yielded spurious free T₄ values that correlated directly with serum protein concentrations.

Conclusion: The presence or absence of T₄-binding proteins in dialysates and ultrafiltrates, and the spurious free T₄ values that these proteins cause can be documented using a nonanalog free T₄ RIA.