Endocrine Abstracts (2006) 11 P818

Reference intervals of maternal free thyroxine (FT4) at the second and third trimesters of pregnancy using the Beckman Coulter’s Access® free T4 assay

V Jarrige, H Meurisse & M D’Herbomez

University of Lille, Lille, France.

Pregnancy causes significant physiological changes that can alter the results of thyroid function testing. The changes in circulating plasma-binding protein (TBG) concentrations interfere with FT4 estimate methods, and the hCG thyrotropic effect impacts TSH activity estimation. A TSH assay is the most sensitive test to assess thyroid status. T4 levels have been implicated as associated with fetal neurodevelopment. During pregnancy, differences in FT4 concentrations have been described depending on the method used.

The objective of this study was to establish pregnancy-specific reference intervals for the Access® Free T4 assay. Reference intervals for the Access® HYPERsensitive hTSH assay and for the Access® Free T4 assay were defined in a previous evaluation (Clin. Chem. Lab. Med. 2005, 43(1)102–105).

Concentrations of FT4 were measured in sera from 364 women with no goiter, no history of thyroid dysfunction and on no medication that could interfere with thyroid function: 109 nonpregnant (<40 years), 139 during the 2nd trimester and 116 during the 3rd trimester of pregnancy (mean age of 27 years). Assessment of TPO-antibodies was systematically done and the positive samples were excluded (6%). The Access® Free T4 assay is a two-step competitive assay. FT4 distribution was calculated using the Sharipo-Wilkes test.

In the control group, the central 95% range between the 2.5th and the 97.5th percentiles was 0.4 to 4.2 mIU/L for TSH, and 7.85 to 13.2 pmol/L for FT4 (mean=10.53, S.D.=1.34). A decrease of 24% and 28.3% in FT4 concentrations from nonpregnant levels was observed in the second and in the third trimester, respectively. FT4 trimester-specific reference intervals defined as central 95% range, were 5.84 to 10.2 pmol/L (mean=8.02, S.D.=1.09) and 5.51 to 9.60 pmol/L (mean=7.55, S.D.=1.02) for the 2nd and the 3rd trimester, respectively.

During pregnancy, thyroid function tests should be interpreted according to trimester-specific reference intervals defined for the method used.

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