Rat placenta is an active site of inner ring deiodination of thyroxine and 3,3',5-triiodothyronine.

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 placenta 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.