Type II and type III deiodinase activity in human placenta as a function of gestational age

Thyroid hormones are essential for fetal development. T-4 can be activated by type I (ID-I) and type II (ID-II) iodothyronine deiodinase or inactivated by type III deiodinase (ID-III). The influence of placental ID-II and ID-III on the regulation of fetal thyroid hormone levels was investigated. Using [I-125]T-4 and [I-125]T-3, respectively, ID-II and ID-III activities were measured in homogenates of normal human placentas from 6-43 weeks gestational age and in placentas from five term neonates with a total thyroid hormone synthesis defect; ID-II and ID-III activities related to protein or DNA concentration decreased and total placental ID-III activity increased significantly during pregnancy, whereas the increase in total placental ID-II activity was not significant. Absolute placental ID-Il activity was approximately 200 times lower than ID-III activity at all gestational ages. Therefore, fluctuations in ID-II activity were not likely to have a significant influence on Fetal thyroid hormone concentrations, but may play a role in the regulation of intraplacental T-3 generation. The high ID-III activity most likely influences the thyroid hormone economy of the fetus. Severely hypothyroid newborns showed strongly decreased serum T-4 levels, but serum T-3 and placental ID-III activities were similar to those in euthyroid newborns. These results suggest that placental ID-ID activity is regulated by serum T-3, but not by serum T-3.