Effect of levothyroxine treatment on serum lipids and pregnancy outcomes in pregnant women with isolated hypothyroxinemia

374 pregnant women with isolated hypothyroxinemia (IH) who were ≤ 20 weeks were included retrospectively in this study. Based on the confirmed gestational age and the use of levothyroxine (LT4), the patients were divided into the treated group (T1 group) and untreated group (C1 group) in first trimester (≤ 13^{+ 6} weeks), the treated group(T2 group) and untreated group (C2 group) in second trimester (14-20 weeks). Data on thyroid function and lipid indices was collected both before and after LT4 treatment. To compare the thyroid function, lipid indices and pregnancy outcomes after LT4 treatment. There was a negative correlation between FT4 levels and TC and LDL levels in the first trimester (P<0.05). FT4 and HDL levels in T1 group were significantly increased, and TSH, TC, TG and LDL levels were decreased, compared to C1 group (P < 0.05). FT4 levels in T2 group were higher than C2 group, and there was no significant difference in other indicators. The risk of spontaneous abortion, gestational diabetes mellitus (GDM) and macrosomia in T1 group was significantly decreased (P < 0.05), and there was no significant difference between T2 and C2 groups. Thus, LT4 treatment can improve the level of FT4 and lipids and reduce adverse pregnancy outcomes in women with IH in the first trimester.