Preconception Body Mass Index and Maternal Thyroid Function: A Longitudinal Observational Study

Objective: 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 with obesity (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 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. First-trimester low FT4 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). Conclusions: Higher p-BMI is associated with a persistently less favorable maternal thyroid hormone profile throughout pregnancy. Early-pregnancy low FT4 concentrations showed the strongest association with obstetric risk, supporting further investigation into early thyroid assessment in higher-risk populations.