Thyroid Problems in the Baby Rarely, a baby may be born without a thyroid gland. This birth defect is not caused by thyroid problems in the mother. Therefore, all newborn babies in the United States routinely have a blood test to be sure that hypothyroidism is diagnosed and treated. Most thyroid medications will have no effect on the baby. The exception to this generality is the administration of radioactive iodine to the mother during pregnancy.
Radioactive iodine can cross the placenta and it can destroy thyroid cells in the fetus.
Much of this information was obtained from a patient information pamphlet produced and distributed by the American Thyroid Association . Their expertise is acknowledged and appreciated. Copyright 1997, 1998.