ROLE OF SULFATION IN THYROID-HORMONE METABOLISM

The type I iodothyronine deiodinase (ID-I) in liver and kidney converts the prohormone thyroxine (T-4) by Outer ring deiodination (ORD) to bioactive 3,3',5-triiodothyronine (T-3) or by inner ring deiodination (IRD) to inactive 3,3',5(-)-triiodothronine (rT(3)), while it also catalyzes the IRD of T-3 and the ORD of rT(3), with the latter as the preferred substrate. Sulfation of the phenolic hydroxyl group blocks the ORD of T-4, while it strongly stimulates the IRD of both T-4 and T-3, indicating that sulfation is an important step in the irreversible inactivation of thyroid hormone. This review summarizes recent studies concerning this interaction between sulfation and deiodination of iodothyronines, the characterization of iodothyronine sulfotransferase activities, the measurement of iodothyronine sulfates in humans and animals, and the possible physiological importance of iodothyronine sulfation.