Thyroid disease and gestational diabetes mellitus (GDM) are frequent complications during pregnancy. We observed the relationship between thyroid indicators and blood glucose to analyze whether thyroid function is associated with the development of GDM. We enrolled a total of 575 pregnant women diagnosed with GDM and 573 pregnant women without GDM. The correlation between thyroid indicators and blood glucose levels was established through correlation analysis. In addition, stratified analysis and restricted cubic spline curves were employed to describe the association between thyroid indicators and the incidence of GDM. We found no significant difference in urine iodine levels between the GDM and non-GDM groups throughout the second trimester. The levels of free triiodothyronine (FT3) and both fasting blood glucose and post-load blood glucose showed a robust positive connection. Thyroid-stimulating hormone (TSH) and free thyroxine (FT4), on the other hand, showed a weakly positive connection with these glucose values. A nonlinear correlation between FT3 and the risk of GDM was also found (pNonlinear=0.0007, p<
0.0001). Particularly, those in the top quartile of FT3 had a 6.99-fold greater risk than those in the lowest. Notably, FT3 levels below 4.04 pmol/l were linked to a decreased chance of developing GDM, but levels over 4.04 pmol/l were linked to a greater risk. Our study successfully established the correlation between thyroid indicators and the risk of GDM. Notably, we discovered a non-linear association between FT3 levels and GDM. The study suggests that ensuring optimal thyroid function during pregnancy may decrease the likelihood of developing GDM.