Abstract: Objective: To estimate the influence of treatment with Ｌ－ｔｈｙｒｏｘｉｎｅ（LT4） on incidence of gestational diabetes mellitus（GDM） in pregnant women with ｓｕｂｃｌｉｎｉｃａｌ ｈｙｐｏｔｈｙｒｏｉｄｉｓｍ（SCH）. Methods: A prospective cohort study of 1 084 gravidas with SCH (SCH group) and 8 848 gravidas with normal thyroid function （ｃｏｎｔｒｏｌ ｇｒｏｕｐ） diagnosed by serum screening presented for prenatal care was performed. SCH group were divided into SCH - treated group and SCH-untreated group according to whether the patients willing to be treated with LT4. Incidences of GDM diagnosed by oral glucose tolerance test（OGTT ） in every group were compared, as well as serum levels of TSH/FT4/ｔｈｙｒｏｉｄ ｐｅｒｏｘｉｄａｓｅ ａｎｔｉｂｏｄｙ（TPOAb）. Results: Serum TSH level of SCH group was higher than control group while serum fT4 level was lower（all Ｐ=０．０00）. Serum TSH levels of either SCH-treated group or SCH-untreated group was higher than control group, while serum fT4 levels was lower （all Ｐ=０．０00）． No significant differences of TSH indicated between SCH-treated group and SCH-untreated group（Ｐ＞０．０５）. Overall positive rate of SCH group is higher than control group （Ｐ=０．０00）, and positive rate of either SCH-treated group or SCH-untreated group was higher than that of control group （P=0.000）. Positive rate of TPOAb showed no significant statistical differences between SCH-treated group and SCH-untreated group（P＞0.05）. Incidence of GDM occurred in SCH group was higher than the normal gravidas, difference was statistically significant（P=0.000）. Incidence of GDM in SCH-treated group was lower than the SCH-untreated group（P=0.035）. Incidence of GDM showed no significant differences between SCH-treated group and control group（Ｐ＞０．０５）, while incidence of GDM in SCH-untreated group was higher than control group （P＜0.01）. Conclusions: It should be paid attention to monitoring and control level of blood sugar in pregnant women with SCH because SCH increases incidence of GDM in gestation period. Using LT4 treatment as soon as possible when diagnosed as SCH after screening of thyroid function, at the same time focus on observing the changes of the blood sugar and insulin levels, as well as preventing GDM through early nutrition and exercise intervention, all above may have a positive role to improve the incidence of GDM in SCH pregnant women and reduce the dangers to mothers and children in patients of SCH combined with GDM.