The fetus which of the following statements is true?

• A. Is dependent on thyroid hormone for normal neurologic development
• B. Does not develop a thyroid gland until the third trimester
• C. Is not susceptible to damage from radioactive iodine therapy given to the mother
• D. Will be born with hypothyroidism in approximately 1 of 400 births in developed countries

Answer: (A)

The key idea is that fetal thyroid hormone is essential for normal brain development. The fetus relies on maternal thyroid hormone that crosses the placenta, especially early in gestation, because the fetal thyroid isn’t fully ready to support neurodevelopment right away. The fetal thyroid begins forming early and starts to function in the first trimester, but maternal hormone continues to play a crucial role until the fetus can regulate its own supply. When maternal thyroid hormone is insufficient or iodine is lacking, the result can be permanent neurodevelopmental impairment, which is why adequate maternal thyroid status is so important for the developing brain.

Radioactive iodine given to the mother can cross the placenta and accumulate in the fetal thyroid, potentially causing lasting damage to the fetal gland and neurodevelopment. The idea that the fetus develops thyroid function only in the third trimester isn’t accurate; thyroid activity starts earlier, and dependence on maternal hormone persists until the fetal thyroid can fully take over. As for prevalence, congenital hypothyroidism is a serious concern, but in developed countries it occurs roughly in the range of a few per thousand births (about 1 in 2,000 to 4,000), not as high as 1 in 400.