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Clinical Trial
. 2009 Mar;200(3):260.e1-6.
doi: 10.1016/j.ajog.2008.10.042. Epub 2008 Dec 27.

Free T4 immunoassays are flawed during pregnancy

Richard H Lee  1 , Carole A Spencer, Jorge H Mestman, Erin A Miller, Ivana Petrovic, Lewis E Braverman, T Murphy Goodwin
Affiliations
Clinical Trial

Free T4 immunoassays are flawed during pregnancy

Richard H Lee et al. Am J Obstet Gynecol. 2009 Mar.

Abstract

Objective: The purpose of this study was to evaluate the diagnostic accuracies of 2 free thyroxine immunoassays during pregnancy.

Study design: Serum was collected from healthy, thyroid peroxidase antibody-negative women during each trimester and nonpregnant controls. Thyrotropin, total T4 (TT4), free T4 index (FT4I), and 2 different FT4 immunoassays were studied.

Results: As expected, TT4 was elevated in all 3 trimesters compared to controls (P < .001). FT4I was elevated in the 1st trimester as compared with controls (P < .05) and returned to the nonpregnant range in the 2nd and 3rd trimesters. In contrast, 1st trimester FT4 immunoassay values were either comparable or lower than controls and by the 2nd and 3rd trimesters had decreased to approximately 65% of controls.

Conclusion: Neither FT4 immunoassay accurately reflects established free T4 changes during pregnancy. TT4 and the FT4I retained an appropriate inverse relationship with TSH throughout pregnancy and appear to provide a more reliable free T4 estimate.

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Comment in

  • Free thyroxine assays: no going back!
    Midgley JE, Christofides ND. Midgley JE, et al. Am J Obstet Gynecol. 2010 Feb;202(2):e6; author reply e6. doi: 10.1016/j.ajog.2009.08.033. Epub 2009 Nov 4. Am J Obstet Gynecol. 2010. PMID: 19889380 No abstract available.