Thyroid hormone is needed for the healthy development of the fetus, particularly the development of the nervous system in the newly-conceived embryo. Lack of available thyroid hormone in the mother's blood supply can lead to severe physical and mental retardation. For this reason, thyroid hormone testing has been recommended for women of childbearing age, even prior to conception.
After the birth of the baby, almost all states require thyroid hormone testing to be performed in the newborn. Babies who are born without thyroid glands, or with insufficient production of thyroid hormone, are prone to diminished cognitive mental capacity.