The development of the thyroid gland in embryogenesis. An embryo of the thyroid gland arises in the 3-4th week of pregnancy as a protrusion of the ventral wall of the pharynx between I and II pairs of gill pockets at the base of the tongue. From this protrusion, the thyroid-lingual duct is formed, which then turns into an epithelial cord, growing down along the anterior intestine. By the 8th week, the distal end of the strand bifurcates (at the level of III-IV pairs of gill pockets); from it subsequently the right and left thyroid lobes are formed, located in front and on the sides of the trachea, on top of the thyroid and cricoid cartilage of the larynx. The proximal end of the epithelial cord normally atrophies, and only the isthmus that connects both lobes of the gland remains from it. The thyroid gland begins to function at the 8th week of pregnancy, as evidenced by the appearance thyroglobulin in fetal serum. At the 10th week, the thyroid gland acquires the ability to capture   iodine . By the 12th week, the secretion of thyroid hormones and the storage of colloid in the follicles begins . Starting from the 12th week, the concentrations of TSH ,       thyroxin-binding globulin , total and free   T4 , total and free T3 in the fetal serum gradually increase and reach levels typical for adults by the 36th week.   

Thyroid hormones in the mother and fetus: 

– The thyroid hormones of the mother pass into the fetal blood through the placenta. Up to 50% of T4 in the serum of the newborn can be represented by maternal T4. Thus, maternal thyroid hormones are involved in the regulation of fetal development even before the maturation of its own hypothalamic-pituitary-thyroid system. Moreover, maternal hormones protect the fetus from hypothyroidism up to childbirth (but do not provide a normal concentration of T4 in the fetal blood). There is another mechanism of intrauterine protection against hypothyroidism – the acceleration of the conversion of T4 to T3 due to the increased activity of 5′-deiodinase in the brain of the fetus .      

– Hypothyroidism in women often leads to anovulatory cycles and infertility . Therefore, the frequency of hypothyroidism in pregnant women is only 0.3%. Pregnancy in women with hypothyroidism in almost 50% of cases ends in a miscarriage . Abortion can be prevented with levothyroxine replacement therapy . If hypothyroidism is not treated during pregnancy, the child may experience mental retardation .          

Treatment of hypothyroidism in the fetus . Hypothyroidism in the fetus is rare and may be due to agenesis or dysgenesis thyroid , hereditary defects of thyroid hormone synthesis , autoimmune thyroid disease in the mother (due to penetration tireoblokiruyuschih antibodies across the placenta) treating hyperthyroidism ( Graves’ disease ) the mother antithyroid agents or radioactive iodine . The presence of hypothyroidism in the fetus can be proved by measuring the content of TSH , T4 or T3 in cord blood. It has been proven that injections of levothyroxine (250-500 μg weekly until delivery) into the amniotic cavity reduce the size of the goiter in the fetus and normalize the concentration of T4 and TSH in umbilical cord blood.