First- and second-trimester thyroid hormone reference data in pregnant women: a FaSTER (First- and Second-Trimester Evaluation of Risk for aneuploidy) Research Consortium study

Objective: The purpose of this study was to calculate first and second trimester reference ranges and within-woman correlations for TSH, free T4, and thyroid antibodies. Study Design: TSH, free T4, and thyroid antibodies were measured in paired sera from 9562 women in the FaSTER trial of Down syndrome screening. Results: The median first trimester TSH (1.05 mIU/L) is lower than the second (1.23 mIU/L); and 98th centile is higher (4.15 vs 3.77 mIU/L). Within-woman paired TSH correlations are moderately strong (r² = 0.64). Among women with first trimester TSH values above the 98th centile, second trimester values are over the 95th centile in 68%. Median first trimester free T4 values (1.10 ng/dL) are higher than second (1.01 ng/dL). Paired free T4 measurements correlate weakly (r² = 0.23). Among women with first trimester free T4 values below the 2nd centile, second trimester values are below the 5th centile in 32%. Antibody measurements correlate strongly between trimesters (thyroperoxidase r² = 0.79, thyroglobulin r² = 0.83). Conclusion: TSH and free T4 measurements require gestation-specific reference ranges.