Intra-amniotic injection of thyroxine (T4Y to a human fetus. Evidence for conversion of T4 to reverse T3.

131I (150 mCi) was inadvertently given to a woman during week 10 to 11 of her gestation. When referred, fetal size was estimated at 31 to 32 weeks' gestation. Because of the potential risk of fetal hypothyroidism, an amniocentesis, with an injection of 500 mug of thyroxine, was performed weekly from week 33 until delivery. T4, T3, thyrodi-stimulating hormone (TSH), and rT3 were measured in amniotic fluid samples obtained at 33, 36, and 37 weeks. Maternal serum T4 was measured on the day of delivery. T4, T3, and TSH concentrations were measured in cord blood and during the neonatal period. The concentration of T4 in amniotic fluid (AF) was within the normal range at 32 weeks (prior to thyroxine); AF T3 and TSH were not detectable. The concentration of AF rT3 at 32 weeks also was normal (160 ng. per deciliter) and increased markedly after beginning intra-amniotic T4 injections; AF T4 increased modestly, but the AF T3 level remained unmeasurable. The concentration of T4 in cord serum obtained within 24 hours of the last amniotic fluid thyroxine injection, was in the hyperthyroid range and the TSH level was low. The cord serum T3 level was at the upper range of normal. The infant's serum T4 and T3 both increased during the first 24 hours. The male infant developed normally and serum T4 and TSH concentrations were normal at 4 months of age. The data indicate: (1) that T4 injected into AF is absorbed by the human fetus, (2) that AF rT3 concentration increases markedly after the AF T4 injection, whereas AF T3 levels do not increase, and (3) that the neonatal TSH surge is not entirely suppressed by hyperthyroid levels of cord serum T4.