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Clinical Chemistry 47: 1353-1363, 2001;
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(Clinical Chemistry. 2001;47:1353-1363.)
© 2001 American Association for Clinical Chemistry, Inc.

Reviews

Direct and Indirect Free Thyroxine Assay Methods: Theory and Practice

John E.M. Midgleya

aNorth Lakes Clinical, 6 High Wheatley, Ilkley, West Yorkshire LS29 8RX, United Kingdom. E-mail je.midgley{at}virgin.net.co.uk.

Background: For the diagnosis of thyroid disease, measurement of "free hormone" is generally accepted as an appropriate measure. However, valid assays measuring the free fraction of thyroxine (FT4) ideally must perform without bias, despite large variations in the concentrations and affinities of serum T4-binding proteins in the population. Several approaches have been taken to overcome such bias, and these have created considerable controversy in the field over the past decade.

Approach: This review, from both a historical and an analytical standpoint, charts the progress made over more than 30 years in improvements to the performance of assays in common use for the measurement of FT4 in serum or plasma. It reexamines the theory behind early approaches to such assays [for example, the free thyroxine index (FTI) method], that preceded more accurate, two-step immunoassays or one-step analog techniques. It evaluates the continuous refinements to the latter assays that by now have largely supplanted the FTI approach and where the deficiencies that so exercised clinical chemists in the past have been virtually eliminated in the leading assays.

Content: The basic Mass Action theory underpinning all such methods is discussed by assessing how far each particular approach obeys the criteria the theory imposes. In this, it is not the intention of the review to dissect individual commercial or academic assays, but rather to give guidance where appropriate as to how any assay said to measure FT4 can be conveniently evaluated by those intending to use it. Examples are given where inappropriate tests may wrongly imply assay invalidity by misinterpreting how FT4 assays work.

Summary: Detailed knowledge of the underlying theory is essential when devising tests for direct FT4 assays, to ensure that such tests do not overstep the practical limits of assay validity.




The following articles in journals at HighWire Press have cited this article:


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B. Yue, A. L. Rockwood, T. Sandrock, S. L. La'ulu, M. M. Kushnir, and A. W. Meikle
Free Thyroid Hormones in Serum by Direct Equilibrium Dialysis and Online Solid-Phase Extraction-Liquid Chromatography/Tandem Mass Spectrometry
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J. E.M. Midgley
Spurious Conclusions on Analog Free Thyroxine Assay Performance
Clin. Chem., September 1, 2007; 53(9): 1714 - 1714.
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R. Sapin and M. d'Herbomez
Free Thyroxine Measured by Equilibrium Dialysis and Nine Immunoassays in Sera with Various Serum Thyroxine-binding Capacities
Clin. Chem., September 1, 2003; 49(9): 1531 - 1535.
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Circulating Soluble Leptin Receptor and Free Leptin Index during Childhood, Puberty, and Adolescence
J. Clin. Endocrinol. Metab., October 1, 2002; 87(10): 4587 - 4594.
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