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

Human placental homogenate deiodinates the tyrosyl ring of T4 and T3, converting these active thyroid hormones to the inactive iodothyronines, rT3 from T4, and 3,3'-diiodothyronine and 3'-monoiodothyronine from T3. The conversion of T4 or rT3 is time, temperature, pH, and protein content dependent and does not occur in the absence of the thiol-regenerating agent dithiothreitol. Phenolic ring deiodination of T4, T3, and rT3 was not detected. Failure of the transplacental passage of T4 and T3 from mother to fetus may be secondary to the placental tyrosyl ring deiodination of these iodothyronines.