Transplacental transfer of maternal T4 during gestation primarily protects which aspect of neonatal development?

• A. Cardiac development
• B. Lung development
• C. Kidney development
• D. Brain development

Answer: (D)

Transplacental transfer of maternal T4 primarily protects brain development because early in gestation the fetal thyroid isn’t fully functional, so the brain relies on maternal thyroid hormone. Maternal T4 crosses the placenta and is available to the developing brain, where it is converted to T3 and drives neuronal proliferation, migration, differentiation, and myelination. Without adequate maternal thyroid hormone, fetal brain development can be severely impaired, leading to irreversible neurodevelopmental deficits. Other organs are important too, but the most critical and sensitive impact of maternal T4 during this period is on brain development.