T 4 levels in the cord blood of athyroid fetuses have been shown to be about one-third of maternal levels. Nevertheless, maternal-fetal transfer of T 4 may not prevent in utero hypothyroidism. Hypothyroidism during pregnancy is associated with a higher rate of complications, including spontaneous abortion and preeclampsia, and has been reported to have an adverse effect on fetal and childhood development. On the basis of current knowledge, levothyroxine sodium should therefore not be discontinued during pregnancy, and hypothyroidism diagnosed during pregnancy should be treated. Studies have shown that during pregnancy T 4 concentrations may decrease and TSH concentrations may increase to values outside normal ranges.
Postpartum values are similar to preconception values.
Elevations in TSH may occur as early as at 4 weeks gestation. Pregnant women who are maintained on levothyroxine sodium should have their TSH measured periodically. An elevated TSH should be corrected by an increase in levothyroxine sodium dose. After pregnancy, the dose can be decreased to the optimal preconception dose. Nursing Mothers Minimal amounts of thyroid hormones are excreted in human milk.