Type II and type III deiodinase activity in human placenta as a function of gestational age.
Thyroid hormones are essential for fetal development. T4 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 [125I]T4 and [125I]T3, 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-II 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 T3 generation. The high ID-III activity most likely influences the thyroid hormone economy of the fetus. Severely hypothyroid newborns showed strongly decreased serum T4 levels, but serum T3 and placental ID-III activities were similar to those in euthyroid newborns. These results suggest that placental ID-III activity is regulated by serum T3, but not by serum T4.
Koopdonk-Kool
J M
JM
Department of Pediatrics and Neonatology, Academic Medical Center, University of Amsterdam, The Netherlands.
de Vijlder
J J
JJ
Veenboer
G J
GJ
Ris-Stalpers
C
C
Kok
J H
JH
Vulsma
T
T
Boer
K
K
Visser
T J
TJ
D005260:Female; D005315:Fetal Diseases / Q000201:enzymology; D005865:Gestational Age; D006801:Humans; D007037:Hypothyroidism / Q000201:enzymology; D007453:Iodide Peroxidase / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy
D007527:Isoenzymes; D007453:Iodide Peroxidase