Fig. 1.

Principal enzymatic activities of the deiodinases type 1, 2, and 3. Removal of iodine (purple spheres) from the outer ring of T4 (3,5,3′,5′-tetraiodothyronine) by D1 and D2 converts T4 to the active hormone T3 (3,5,3′-triiodothyronine), with D2 having a higher affinity for T4 than D1. Inner-ring deiodination of T4 and T3 by D3 generates the biologically inactive metabolites reverse T3 (3,3′,5′-triiodothyronine) and T2 (3,3′-diiodothyronine), respectively