Low Thyroid Hormone in Early Pregnancy Is Associated With an Increased Risk of Gestational Diabetes Mellitus.

CONTEXT: Although thyroid dysfunction in early pregnancy may have adverse effects on pregnancy outcome and offspring, few prospective studies have evaluated these effects.

OBJECTIVE: Our aim was to evaluate the correlations between different thyroid hormone levels in early pregnancy and the incidence of gestational diabetes mellitus (GDM).

SETTING AND PARTICIPANTS: 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.

MAIN OUTCOME MEASURES: We focused on GDM during the index pregnancy.

RESULTS: 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 T4 (FT4 ) in early pregnancy in GDM women was lower than that in non GDM women (P < .0001). With increasing early pregnancy FT4 , the rate of incident GDM was decreasing (P < .0001).

CONCLUSIONS: Low thyroid hormone levels in early pregnancy are a risk factor for GDM incidence.