FT3

This study investigated the relationship between thyroid markers in early pregnancy and gestational diabetes mellitus (GDM) risk, with a focus on potential mediation through lipid species. Among 6068 pregnant women, higher free thyroxine (fT4) levels were associated with a reduced risk of GDM (OR = 0.73, 95% CI 0.54–0.98, Ptrend = 0.043), while a higher fT3/fT4 ratio was linked to an increased GDM risk (OR = 1.43, 95% CI 1.06–1.93, Ptrend = 0.010). Lipidomic analysis in a subset of participants revealed that the fT3/fT4 ratio was positively associated with specific lipid species, including alkylphosphatidylcholine 36:1 and phosphatidylcholine 40:7. Mediation analysis showed that 67.9% of the relationship between the fT3/fT4 ratio and GDM was mediated through these lipids. These findings suggest that lower fT4 levels and higher fT3/fT4 ratios in early pregnancy are significant risk factors for GDM, with lipid metabolism playing a key mediating role.