Thyroid markers in early to mid-pregnancy and risk of gestational diabetes: a longitudinal retrospective cohort study.

BACKGROUND

Thyroid markers have been associated with an increased risk of gestational diabetes mellitus (GDM), but existing evidence remains inconsistent. The purpose of this study was to examine the associations between free T3 (FT3), thyroid autoimmunity, FT3/free T4 (FT4) ratio, and gestational diabetes mellitus (GDM) in a Chinese population.

METHODS

This longitudinal cohort study enrolled 10, 258 women at Shanghai First Maternity and Infant Hospital between January 2023 and November 2025, of whom 7, 463 were included in the final analysis. Thyroid function was assessed at 4-24 gestational weeks, and GDM was diagnosed by a 75-g oral glucose tolerance test (OGTT) at 24-28 weeks, with GDM as the primary outcome.

RESULTS

In early (4-12 weeks) and mid-pregnancy (13-24 weeks), FT3 and the FT3/FT4 ratio were both positively associated with GDM. Lipid metabolism differed significantly across percentiles of the FT3/FT4 ratio. Thyroid peroxidase antibody (TPOAb) was associated with GDM only in early pregnancy, whereas thyroglobulin antibody (TGAb), FT4, and TSH showed no significant associations with GDM.

CONCLUSION

A higher FT3/FT4 ratio in early and mid-pregnancy was associated with abnormal glucose and lipid metabolism and was positively associated with GDM risk, suggesting its potential role as a supplementary marker for early risk stratification of GDM.