Preconception body mass index and maternal thyroid function: a longitudinal observational study

Objective: The objective of this study was to examine the association between preconception body mass index (p-BMI) and maternal thyroid function longitudinally evaluated over pregnancy and to explore whether early-pregnancy thyroid parameters are associated with adverse obstetric outcomes. Methods: Using preconception BMI as a stratification variable, 1,107 pregnant women were classified as normal weight (NW), overweight (OW), or obese (OB). Thyroid function was assessed at ≤13 gestational weeks and at two subsequent time points. Thyrotropin (TSH) and free thyroxine (FT4) longitudinal trajectories were analyzed using generalized estimating equations, and the FT4-TSH relationship was evaluated by correlation analyses. As an exploratory analysis, logistic regression models were estimated to assess associations between first-trimester thyroid parameters and a composite adverse obstetric outcome (preterm delivery, labor dystocia, premature rupture of membranes, placental abruption, or abnormal amniotic fluid volume). Results: At early pregnancy, OW and OB women exhibited lower FT4 and higher TSH concentrations than NW women. Across gestation, FT4 declined and TSH increased in all p-BMI categories, with persistently lower FT4 and higher TSH levels in women with higher p-BMI and no significant p-BMI × gestational period interaction. Low FT4 in the first trimester was independently associated with adverse obstetric outcomes (odds ratio (OR) = 2.04, 95% CI: 1.32–3.11, P = 0.001), whereas the association with elevated TSH was weaker (OR = 1.71, 95% CI: 1.04–2.77, P = 0.03). Conclusion: Higher p-BMI is associated with a persistently less favorable maternal thyroid hormone profile throughout pregnancy. Low FT4 concentrations during early pregnancy showed the strongest association with obstetric risk, supporting further investigation into early thyroid assessment in higher-risk populations.