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Most authorities consider that thyroid hormones do not readily cross the placenta. Placental transfer has been reported, but in amounts so limited that a mother with physiological concentrations of levothyroxine sodium and tri-iodothyronine would not provide normal thyroid hormone concentrations to a fetus with congenital hypothyroidism.
Minimal amounts of thyroid hormones are distributed into breast milk. There is sufficient thyroid hormone to meet the biological needs of a sucking infant with a nonfunctioning thyroid gland but it has been suggested that levothyroxine sodium in breast milk might mask any hypothyroidism in the sucking newborn.
