This study will be conducted to determine the effectiveness of the effect of Levothyroxine on pregnancy outcome of pregnant women with subclinical hypothyroidism and positive thyroid peroxidase antibody (TPOAb) of two parts: in the first stage thyroid hormones are tested in 1600 pregnant women in the first 20 weeks of pregnancy and Pregnant women with subclinical hypothyroidism and positive TPOAb are identified. Three times urine during a week are also sampled for iodine measurement. Women with TPOAb positive (more than 50 kIU/l) and TSH between 2.5 and 10 mIU/ liter in the first 20 weeks of pregnancy, will enter into the second phase of the study. In the second stage of study, pregnant women with subclinical hypothyroidism and positive TPOAb are divided simple randomly (based on the odd or even sample code) into two groups. The first group, intervention group, is treated with levothyroxine. The second group, control group, received placebo. In the second and third trimester, maternal serum samples are taken. Both groups are followed up until delivery and abnormal cases are recorded. In the second and third trimester, women’s serum samples are evaluated for TSH, T4 and T3uptake and newborns’ serum samples are collected for TSH analysis. The intervention groups is compared with control group according to pregnancy outcome including: abortion, preterm labor, low birth weight, intrauterine growth restriction, premature rupture of membrane, third trimester hemorrhage and newborns’ thyroid hormone levels at birth.