Maternal to fetal thyroxine transmission in the human term placenta is limited by inner ring deiodination.
Placental deiodination of T4 to rT3 has been proposed as the factor controlling materno-fetal transmission of T4. We investigated T4 transfer in the isolated perfused human placental lobule with and without addition of the deiodinase inhibitor, iopanoic acid. T4 (150 nmol/L) in protein-free medium was added to the maternal circuit. Without iopanoic acid, the appearance of T4 in the fetal circuit was very low, with fetal T4 levels reaching only 4.1 +/- 0.84 pmol/L at 6 h. Levels of rT3 rose progressively in both circuits, reaching 28.8 +/- 5.5 nmol/L in the maternal and 12.4 +/- 3.2 nmol/L in the fetal circuit by 6 h. No T3 could be measured in either circuit. Addition of 0.5 nmol/L iopanoic acid to maternal perfusate, however, resulted in significant reduction in the appearance of rT3 [maternal levels, 0.58 +/- 0.06 nmol/L (2% of control values); fetal levels, 0.33 +/- 0.03 nmol/L (2.7% of control values)] and a major (approximately 2700-fold) increase in T4 appearance in the fetal circuit, with fetal T4 levels reaching 10.1 +/- 3.4 nmol/L at 6 h. These results support the hypothesis that placental inner ring (type III) deiodination is a major factor controlling placental transmission of maternal T4.
Mortimer
R H
RH
Conjoint Endocrine Laboratory, Royal Brisbane Hospital, Brisbane, Queensland, Australia.
Galligan
J P
JP
Cannell
G R
GR
Addison
R S
RS
Roberts
M S
MS
D005260:Female; D006207:Half-Life; D006801:Humans; D066298:In Vitro Techniques; D007453:Iodide Peroxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007480:Iopanoic Acid / Q000494:pharmacology; D008431:Maternal-Fetal Exchange; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D013974:Thyroxine / Q000378:metabolism*
D007453:Iodide Peroxidase; D007480:Iopanoic Acid; D013974:Thyroxine