Association Between Maternal Thyroid Function in Early Pregnancy and Gestational Diabetes: A Prospective Cohort Study

Context: Previous studies on the relationship between thyroid gland function and the development of gestational diabetes mellitus (GDM) have reported different results, leading to the need for a cohort study design with a large sample size.Objective: We aimed to investigate the relationship between thyroid function in early pregnancy and GDM.Methods: This was a prospective cohort study based on the China Birth Cohort Study (CBCS), from February 2018 to December 2020. The study took place at a tertiary maternal and child health hospital. A total of 36 256 pregnant women were successfully recruited based on the CBCS. The main outcome measure was GDM.Results: This study consisted of 26 742 pregnant women who met the inclusion criteria, of whom 3985 (14.90%) were diagnosed with GDM, and the women with GDM were older than their healthy counterparts (33.26 +/- 4.01 vs 31.51 +/- 3.76 years, P < .001). After removing potential influencing variables, we found that increased thyroid-stimulating hormone (TSH) (adjusted odds ratio [aOR] 1.030, 95% CI 1.007, 1.054, P = .012) and subclinical hypothyroidism (aOR 1.211, 95% CI 1.010, 1.451, P = .039), but not free thyroxine or thyroid peroxidase antibody, were associated with the occurrence of GDM. Further analysis indicated a nonlinear relationship between TSH and GDM (P < .05): when TSH <= 1.24 mIU/L, the occurrence of GDM was elevated with increasing TSH, but when TSH > 1.24 mIU/L, this trend was not obvious.Conclusion: High TSH might be associated with increased risk of GDM.