Rat Placenta Is an Active Site of Inner Ring Deiodination of Thyroxine and 3,3′,5-Triiodothyronine*

Abstract

Inner ring deiodination of L-T₄ and L-T₃ by rat placental homogenates resulted in the generation of rT₃ from T₄ and 3,3′-diiodothyronine and 3′-monoiodothyronine from T₃. Dithiothreitol is required in the incubation medium. There was little or no detectable outer ring deiodination of T₄ and T₃. These findings suggest that placental enzymatic inner ring monodeiodination of T₄ and T₃ could prevent the transplacental passage of T₄ and T₃ from dam to fetus. They also provide an explanation for our previous observations that fetal serum rT₃ is partially dependent on maternal thyroid function.