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Clinical Chemistry 53: 911-915, 2007. First published March 15, 2007; 10.1373/clinchem.2006.083915
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Right arrow Endocrinology and Metabolism
(Clinical Chemistry. 2007;53:911-915.)
© 2007 American Association for Clinical Chemistry, Inc.

Endocrinology and Metabolism

A Direct Free Thyroxine (T4) Immunoassay with the Characteristics of a Total T4 Immunoassay

Kristofer S. Fritz1, R. Bruce Wilcox1 and Jerald C. Nelson2,a

Departments of1 Biochemistry and 2 Internal Medicine and Pathology, Loma Linda University School of Medicine, Loma Linda, CA.

aAddress correspondence to this author at: 11234 Anderson Street, Room 1568, Loma Linda, CA 92354. Fax 909-558-0490; e-mail jcnelson{at}llu.edu.

Background: Direct free thyroxine (T4) measurements have been linked to both T4-binding serum protein concentrations and protein-bound T4 concentrations. Whether this is evidence of a relationship to total T4 concentrations has not been reported.

Methods: We compared an analog-based direct free T4 immunoassay and a total T4 immunoassay. Each assay was applied to the fractions of serum T4 obtained by ultrafiltration and equilibrium dialysis. Both were applied to serum-based solutions in which free T4, T4-binding proteins, protein-bound T4, and total T4 were systematically varied, held constant, or excluded.

Results: Neither the free T4 assay nor the total T4 assay detected dialyzable or ultrafilterable serum T4. Both assays detected and reported the T4 retained with serum proteins. Both free and total T4 results were related to the same total T4 concentrations in the presence and absence of T4-binding proteins. Both results were similarly related to total T4 concentrations when free T4 was held constant while total T4 was varied. Both were similarly related to a total T4 concentration that was held constant while free T4 progressively replaced protein-bound T4. These free T4 results, like total T4 results, were unresponsive to a 500-fold variation in dialyzable T4 concentrations.

Conclusion: New experiments extend the characterization of a longstanding and incompletely characterized analog-based free T4 immunoassay. These free T4 measurements relate to total T4 concentrations in the same way that total T4 measurements do.




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K. S. Fritz, R. M. Weiss, J. C. Nelson, and R. B. Wilcox
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Clin. Chem., July 1, 2007; 53(7): 1384 - 1385.
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