Part of “CHAPTER 47 – THYROID DISORDERS OF INFANCY AND CHILDHOOD“

In the human fetus, the period of maturation of thyroid function extends throughout gestation and into the neonatal period.1,2 Thyrotropin-releasing hormone has been detected in the human fetal hypothalamus at 10 to 12 weeks of gestation, and levels increase to term. Pituitary thyroid stimulating hormone (TSH) is detectable at 8 to 10 weeks of gestation, remains low until 16 to 18 weeks, and then increases to a plateau level by 28 weeks of gestation. Human fetal serum TSH is detectable by 10 weeks of gestation and increases to a mean level of ˜15 μU/mL between 20 and 30 weeks. Fetal serum total thyroxine (T₄) levels are low before 16 to 18 weeks of gestation and, thereafter, progressively increase to levels of 10 to 12 μg/dL at 34 to 36 weeks of gestation. By 40 weeks, the mean serum fetal TSH decreases to a mean value of ˜10 μU/mL, indicating maturation of the feedback mechanism.