Associations between maternal thyroid hormones and neonatal outcomes during pregnancy

Introduction This study aims to explore the gestational dynamics of maternal thyroid hormones and their associations with neonatal birth outcomes (birth weight, length, and parity), including 2,884 pregnant women (287 in early, 807 in mid, and 1,487 in late pregnancy). Methods Using univariate ANOVA to assess trimester-specific thyroid hormone differences, linear mixed-effects models to examine associations with neonatal outcomes, restricted cubic spline (RCS) plots to evaluate dose-response relationships, and Bayesian kernel machine regression (BKMR) to analyze combined effects of thyroid hormone mixtures. Results We found that maternal Thyroid Stimulating Hormone (TSH), Total Triiodothyronine (TT3), Free Triiodothyronine (FT3), and Free Thyroxine (FT4) levels changed significantly with gestation (TSH increasing gradually, TT3 peaking in mid-pregnancy, and FT3/FT4 declining continuously), Weak correlations were observed between maternal thyroid hormones and neonatal outcomes, with mixed linear regression revealing trimester- and sex-specific associations of TT4, TT3, FT3, and FT4 with birth weight, length, and parity. RCS analysis further showed dose-response relationships with pregnancy-stage and sex-specific patterns, while BKMR indicated weak positive correlations of thyroid hormone mixtures with neonatal outcomes in early/mid-pregnancy and significant positive correlations in late pregnancy. The relative contribution of each thyroid hormone to neonatal outcomes was also quantified. Conclusion Collectively, our findings highlight trimester-specific thyroid hormone dynamics, complex dose-response relationships with neonatal outcomes, and stage/sex-dependent associations, underscoring the need for gestational thyroid function monitoring to inform maternal–infant health strategies.