Objective:To investigate the effect of iodine nutrition status and sorts of indexes on homocysteine (HCY) in the second trimester of pregnancy under conditions with normal thyroid function.Methods:481 second-trimester pregnant (13th to 27th week of pregnancy) women with normal thyroid function were selected for the study. Urinary iodine concentration (UIC), thyroid auto-antibody, HCY and biochemical indexes were measured. The HCY levels were compared among subjects with different iodine nutritional status, and factors related to HCY level were analyzed.Results:Patients were stratified into iodine deficiency, iodine adequate, iodine more than adequate and iodine excess groups and the proportion were 57.0% ( n=274), 29.7% ( n=143), 10.8% ( n=52) and 2.5% ( n=12), respectively. The overall median UIC was 134.1 μg/L. There was significant difference in HCY levels between iodine excess group and iodine adequate group(1.83 μmol/L vs. 2.46 μmol/L, P=0.036). Spearman correlation analysis showed that HCY was negatively correlated with iodine excess, free triiodothyronine (FT3), thyroglobulin antibody, and fasting blood glucose( r=-0.101, P=0.026; r=-0.099, P=0.03; r=-0.192, P<0.01; r=-0.099, P=0.03), and was positively correlated with TPOAb ( r=0.177, P<0.01). Multiple linear regression analysis showed that HCY was independently negatively correlated with iodine excess (β=-1.505, P=0.043) and FT3 (β=-0.661, P=0.008). Conclusion:Up to 57% women in the second trimester of pregnancy are with iodine deficiency and normal thyroid function. Moderate iodine excess and elevated FT3 levels are beneficial to the decrease of HCY levels and thus reduce the risk of vascular complications in pregnant women.