Background: We aimed to investigate the association of free thyroxin (FT4), free triiodothyronine (FT3), subclinical hypothyroidism (SCH), and thyroid peroxidase antibody (TPOab) in the first trimester with gestational diabetes mellitus (GDM). Methods: We recruited 110 pregnant women with GDM and 100 pregnant women without GDM who had normal 75 g oral glucose tolerance test (OGTT) results between June 2019 and June 2021. We collected basic data from all participants and compared serum FT3 and FT4 levels, SCH, and TPOab (+) incidences in the first trimester between the two groups. We used logistic regression to identify factors that influence the development of GDM. Results: Serum FT3 levels were 4.58 \pm 0.78 and 4.61 \pm1.42 pmol/L in the GDM group and Control group, while FT4 levels were 9.32 \pm 2.54 and 10.24 \pm 2.77 pmol/L. The incidence of SCH were 25.5% and 14.0%, while TPO (+) were 20.0% and 10.0%. The GDM group’s FT4 levels were significantly lower than the control group’s, whereas the GDM group’s age, incidence of SCH, and TPOab (+) were significantly higher (p 0.05). Logistic regression analysis demonstrated that age, SCH and TPO (+) were risk factors for GDM (p 0.05), the regression equation: logit p = –3.484 + 0.105 (age) + 1.128 (SCH) + 1.294 (TPOab (+)). Conclusions: Our findings suggest that monitoring the changes in FT4 levels, SCH, and TPOab (+) incidence in the first trimester may be useful in predicting the occurrence and development of GDM.