Evidence That Triiodothyronine and Reverse Triiodothyronine Are Sequentially Deiodinated in Man*

Abstract

We have demonstrated that in patients given a single iv injection of T₃, rT₃, or, to a lesser extent, T₄, all labeled with ¹²⁵I in the outer or phenolic ring, chromatography of serum on columns of Sephadex G-25 superfine revealed the presence of a labeled material, separate from the administered hormone and from both iodide and iodoprotein. This peak has been termed pre-T₃ because it elutes just before the T₃ peak. Identification of the various compounds in pre-T₃ was carried out by cation exchange chromatography. Pre-T₃ generated from [₁₂₅I]T₃ consistently contained labeled compounds with the chromatographic behavior of 3,3′-diiodothyronine and 3′-monoiodothyronine, while pre- T.j generated from [_l25I]rT₃ contained labeled products with the chromatographic mobility of 3′,5′-diiodothyronine, 3,3'-diiodothyronine, and 3′-monoiodothyronine. In addition, pre-T₃) also contained the glucuro- and sulfoconjugates of these several labeled products. These studies demonstrate that T₃ and rT₃.) undergo progressive and probably sequential deiodination in the peripheral tissues, resulting in the formation of a variety of diiodothyronines and monoiodothyronine, as well as their glucuro- and sulfoconjugates.