The aim of our study was to evaluate the reference ranges of thyrotropin (TSH) and free thyroxin (FT4) assayed with 5 methods at the second and the third trimesters of pregnancy for women with iodine intake at the lower limit of the normal range. This study used 297 sera (42 controls without pregnancy with age <40 years, 139 women at the second trimester and 116 at the third with mean age to 27 years in both groups). The assays were for TSH (Elecsys Roche), for thyroproxidase antibodies (BRAHMS), for FT4 (Dia-Sorin (DS), Elecsys (E), DPC (I), Bayer (ACS), Tosoh biomedical (AIA). Fourteen women (5.5%) have detectable levels of TPOAbs and these sera were eliminated of the study. After to verify the normality or the Log-normality of the distributions, the reference ranges were calculated. By comparison with the controls (1.64 mIU/l), the TSH concentrations rise at the second trimester (1.68 mIU/l) and at the third trimester (1.99 mIU/l, P=0.021 versus C, P=0.004 versus 2 T). The FT4 concentrations decreased during the pregnancy but in variables proportions depending to the assays. We noted a significant mean decrease to 24.8% at the second trimester (extreme 1533) and to 28.7% at the third (extreme 1837). For 3 FT4 methods (DS, E, ACS) the use of appropriated ranges at the third trimester decrease the percentage of values under the normal range in mean from 32% to 2%. For one method all levels were in the normal range however the term and the norms (I). With the last method we noted 52% of low FT4 levels at the third trimester what decreased to 0% with appropriated normal range (AIA). In conclusion, at the second and the third trimester of the pregnancy, the TSH concentrations rise when the FT4 decrease was variable with the assays. Like this, all thyroid results during pregnancy must been interpreted in accordance with reference ranges established for each trimester and each FT4 assays.
01 - 05 Apr 2006
European Society of Endocrinology