A newborn with hypothyroidism has absent thyroid tissue on ultrasound. How can development be normal?

• A. Endogenous fetal thyroid hormone
• B. Maternal T4 crosses placenta
• C. Prenatal T3 therapy
• D. TSH from mother

Answer: (D)

Maternal thyroid hormone crossing the placenta can sustain fetal development even when the fetus has little or no thyroid tissue. The key is that thyroxine (T4) from the mother readily crosses into the fetus and is converted to the active hormone (T3) in fetal tissues, supporting growth and brain development during gestation. Maternal TSH does not cross the placenta, so it cannot rescue the fetus if the thyroid tissue is absent. Prenatal T3 therapy isn’t typically used, and endogenous fetal thyroid production isn’t possible without tissue. After birth, the infant would require thyroid hormone replacement to maintain normal development.