Serum iodothyronines in the human fetus and the newborn: Evidence for an important role of placenta in fetal thyroid hormone homeostasis

The pattern of circulating iodothyronines in the fetus differs from that in the adult, being characterized by low levels of serum T-3. In this study, concentrations of various iodothyronines were measured in sera from neonate s of various postconceptional age (PA). Results obtained in cord sera at bi rth (PA, 24-40 weeks), reflecting the fetal pattern, were compared with tho se found during extrauterine life in newborns of 5 days or more of postnata l life (PA, 27-46 weeks). The main findings are: Starting at 30 weeks of PA , serum levels increase linearly during extrauterine life; and at 40 weeks, they are more than 200% of those measured in cord sera from newborns of eq uivalent PA. Serum reverse T-3 (rT3) levels during fetal life are higher th an those measured during extrauterine life; but they significantly decrease , starting at 30 weeks of PA. Serum T-3 sulfate (T3S) does not significantl y differ between the two groups, showing the highest values at 28-30 weeks of PA, and significantly decreasing at 30-40 weeks. T3S levels are directly correlated with rT3, both in fetal and extrauterine life, whereas a signif icant negative correlation between T3S and T-3 is found only during extraut erine life. In conclusion: 1) changes in serum concentrations of iodothyron ines in umbilical cord and during postnatal life indicate that maturation o f extrathyroidal type I-iodothyronine monodeiodinase (MD) accelerates, star ting at 30 weeks of PA; 2) high levels of type III-MD activity in fetal tis sues prevent the rise of serum T-3, whereas they maintain high levels of rT 3 during intrauterine life; 3) an important mechanism leading to the transi tion from the fetal to the postnatal thyroid hormone balance is a sudden de crease in type III-MD activity; iv) because placenta contains a high amount of type III-MD, it is conceivable that placenta contributes to maintain lo w T-3 and high rT3 serum concentrations during fetal Life and that its remo val at birth is responsible for most changes in iodothyronine metabolism oc curring afterwards.