Background: Pregnancy has a considerable physiological impact on the thyroid gland and its metabolic unction and to meet the increased demands during pregnancy. Thyroid disease is known to impact pregnancy outcomes, and gestational diabetes is the most common obstetric metabolic disease. Both of these conditions can cause short- and long-term harm to the mother and child, and an increasing number of scholars have therefore begun to investigate whether there is a correlation between thyroid disease and GDM Objective: evaluation of the relations between different thyroid hormone levels in early pregnancy and the incidence of gestational diabetes mellitus (GDM). Study design: This prospective observational cross-sectional study was conducted during the January 2018 to June 2018. The study comprised one hundred, euthyroid women with singleton pregnancy who provided early pregnancy serum samples for analyses of thyroid function. GDM was diagnosed using a 2 hours, 75-g oral glucose tolerance test, and the mothers were grouped and compared according to the results. Results: The incidence of GDM in pregnant women tended to increase with age (P< .0001). The level of free T4 (FT4) in early pregnancy in GDM women was lower than that in non GDM women (P< .0001) also found that high maternal weight was associated with a higher GDM rate in the first trimester (P< .0001). Conclusion: Low thyroxine levels in early pregnancy could be a risk factor for GDM development.