There are essentially no risks to being hypothyroid during pregnancy (particularly if you are receving medical care and thyroid hormone replacement). Thyroid hormone does not cross the placenta to affect the developing baby. As an aside, thyroid hormone replacement requirements do not usually change much during pregnancy.

Maternal hypothyroidism should be distinguished from fetal hypothyroidism which can be quite problematic and lead to significant brain damage. In general, the only common way that fetal hypothyroidism occurs is through a lack of sufficient iodine in the diet. (Since the fetus has its own thyroid gland which functions independently of the mother's gland, maternal hypothyroidism does not result in fetal hypothyroidism.) Dietary iodine deficiency is extremely uncommon in the United States but does occur with some frequency in Asia and Africa. All children born in the United States are screened for hypothyroidism at birth. Prompt thyroid supplementation for those who need it seems to prevent long-term injury from neonatal hypothyroidism.

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