Which statement about fetal thyroid development is true?

Which statement about fetal thyroid development is true?

• A. The fetus is dependent on thyroid hormone for normal neurologic development
• B. The fetus does not develop a thyroid gland until the third trimester
• C. The fetus is not susceptible to damage from maternal radioactive iodine therapy
• D. The fetus will be born with hypothyroidism in approximately 1 of 400 births in developed countries

Answer: (A)

Fetal brain development relies on thyroid hormone supplied by the mother early in gestation because the fetal thyroid isn’t fully functional yet. The fetus depends on maternal T4 crossing the placenta to support neural development, and this supply is crucial before the fetal thyroid gland can produce enough hormone on its own (roughly around the first trimester to early second trimester). If maternal thyroid hormone is deficient or there's exposure to factors that disrupt thyroid function, neurodevelopment can be impaired. That’s why the statement is true: the fetus is dependent on thyroid hormone for normal neurologic development. The other points are not accurate: the fetal thyroid begins to function well before the third trimester, so dependence isn’t limited to late pregnancy; maternal radioactive iodine therapy can harm the fetus by suppressing or damaging the developing thyroid; and congenital hypothyroidism occurs at about 1 in 2,000 to 4,000 births in developed countries, not as high as 1 in 400.

Fetal brain development relies on thyroid hormone supplied by the mother early in gestation because the fetal thyroid isn’t fully functional yet. The fetus depends on maternal T4 crossing the placenta to support neural development, and this supply is crucial before the fetal thyroid gland can produce enough hormone on its own (roughly around the first trimester to early second trimester). If maternal thyroid hormone is deficient or there's exposure to factors that disrupt thyroid function, neurodevelopment can be impaired.

That’s why the statement is true: the fetus is dependent on thyroid hormone for normal neurologic development. The other points are not accurate: the fetal thyroid begins to function well before the third trimester, so dependence isn’t limited to late pregnancy; maternal radioactive iodine therapy can harm the fetus by suppressing or damaging the developing thyroid; and congenital hypothyroidism occurs at about 1 in 2,000 to 4,000 births in developed countries, not as high as 1 in 400.