This study will be conducted in two parts: in the first stage thyroid hormones are tested in 1600 pregnant women in the first 20 weeks of pregnancy. It is estimated that 200 pregnant women are identified with thyroid peroxidase antibody (TPOAb) positive. Three times urine during a week are also sampled for iodine measurement. Women with TPOAb positive (more than 50 kIU/l) and TSH less than 2.5 mu/ liter in the first 20 weeks of pregnancy, will enter into the second phase of the study. In the second stage of study, euthyroid women with positive TPOAb are divided randomly into two groups. The first group, intervention group, is treated with levothyroxine. The second group, control group, received placebo. In addition, a group of 100 euthyroid women who are not positive TPOAb is considered as a healthy control group. In the second and third trimester, maternal serum samples are taken. All three groups are followed up until delivery and abnormal cases are recorded. The birth weight, length, head circumference, first and fifth minute apgar score, type of delivery and indications for cesarean and operative delivery 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 groups 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.