In spite of frequent hypertrophy of the gland, the thyroid function remains normal during pregnancy. Of course, the total thyroid hormones level bound to TBG (Thyroxine Binding Globulin) increases regularly as a result of hyperestrogenism, but the level of the free fractions of the thyroid hormones (FT3, FT4) which is normal during the 1st trimester tends to decrease regularly during the 2nd and 3rd trimesters. There is a moderate decrease of the TSH level during pregnancy. The placenta plays a major role in the synthesis and metabolism of the thyroid function hormones. TSH, T3, T4 and thyroglobulin cannot cross the placental barrier while TRH, iodine, and thyrostimulating immunoglobins can. Among medications, thyroid hormones do not cross the placental barrier contrary to synthetic anti-thyroid preparations (SAT), lithium and beta-blockers. Therefore, this should be taken into consideration in the treatment of dysthyroidisms, knowing that the thyroid gland of the fetus becomes functional at 12 weeks: the secretion of fetal T4 and TSH is present as early as the 18-20th week. The thyroid gland of the fetus has no auto-regulation mechanism and the fetus adjust his hormone secretion according to the iodine environment.