Thyroid Physiology in Pregnancy.

Objective. Various physiological changes occur in maternal thyroid economy during pregnancy. This review focuses on the events taking place during gestation and together contributing to deeply influence maternal thyroid function. Methods: Scientific reports on maternal thyroid physiology in pregnancy. Results: During the 1st trimester, hCG induces a transient increase in FT4 levels, which is mirrored by a lowering of TSH concentrations. Following this period, serum FT4 concentrations decrease of ~10-15%, and serum TSH values steadily return to normal. Also starting at early gestation, there is a marked increase in serum thyroxine-binding globulin (TBG) concentrations, which peak around midgestation and are maintained thereafter. This event, in turn, is responsible for a significant rise in total T4 and T3. Finally, significant modifications in the peripheral metabolism of maternal thyroid hormones occur, due to the expression and activity of placental D2 and D3. Conclusion: In line with these variations, both free thyroid hormone and TSH reference intervals change throughout pregnancy, and most scientific societies now recommend that method and gestation-specific reference ranges be used for interpreting results in pregnancy. Maternal iodide pool reduces during pregnancy because of increased renal clearance of iodine and transfer of iodine to the fetal-placental unit. This results in an additional requirement of iodine during pregnancy of ~100% as compared to non-pregnant adults. In accordance, the recommended iodine intake in pregnancy is 250 μg/day. A daily iodine intake below this threshold poses risks for both the mother and the fetus of various degrees of thyroid insufficiency.