The diagnosis and the early treatment of the thyroid diseases occuring before and during pregnancy are essential for both mother and baby. Increased synthesis of the thyroid hormones, urinary iodine loss, and transfer of iodine to the fetus through the placenta increase the daily iodine requirement in pregnant women. In order to evaluate the thyroid functions in pregnant women, the serum free T4 (fT4), free T3 (fT3), and TSH levels should be determined. Low levels of maternal and fetal thyroxin, lead to irreversible central nervous system development defects, where endemic cretinism and mental retardation are the most significant complications. Therefore, if possible, iodine treatment should be initiated prior to pregnancy. On the other hand hyperthyroidism, can lead to complications such as abortion, premature birth, preeclampsia, and placenta detachment. Thus, thyroid function tests should be monitored carefully during pregnancy