Introduction ;thyroid hormones ate important for energy metabolism,thermogenesis,and stimulation of growth and development of various tissues including the central nervous system. In the first trimester , a fetus is completely dependent on maternal thyroid hormones until by the end of the first trimester that the fetus begins to produce thyroid hormone on its own. Association between abnormal thyroid function during pregnancy and adverse effect to the maternal health and child development has been reported. Thyroid peroxidase antibody (TPO-Ab) which found in up to 90% of patients with autoimmune thyroid disease is the most common thyroid abnormality observed in pregnant women. A strong correlation has been found between (TPO-Ab)and risk of spontaneous miscarriage. Furthermore, presence of (TPO-Ab)is associated with postpartum thyroid disease and subsequent impaired development in children. The determination of TPO-Ab in the early gestational age could serve as identification of pregnant at risk. Beside the TPO-Ab, serum thyroid stimulating hormone (TSH) is the most sensitive test for detecting mild thyroid hormone excess or deficiency. Abnormal TSH status in pregnant women has been associated with preterm delivery, fetal death, abruption placenta and impaired neuropsychological development of fetus. The objective of this study was to determine the prevalences of abnormal TSH and (TPO-Ab) inThai pregnancies.