['Although thyroid dysfunction in early pregnancy may have adverse effects on pregnancy outcome and offspring, few prospective studies have evaluated these effects.', 'Our aim was to evaluate the correlations between different thyroid hormone levels in early pregnancy and the incidence of gestational diabetes mellitus (GDM).', 'The study comprised 27 513 mothers 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.', 'We focused on GDM during the index pregnancy.', 'The incidence of GDM in pregnant women tended to increase with age (5.83%, 10.18%, 14.95%, and 22.40%; P < .0001). The incidence of GDM increased with increasing prepregnancy body mass index (P < .0001). Pregnant women with a family history of diabetes had a much higher incidence of GDM than those without a family history of diabetes (21.09% vs 12.92%; P < .0001). The level of free T'] Does low Thyroid Hormone in Early Pregnancy be Associated With an Increased Risk of Gestational Diabetes Mellitus?