title: Extrathyroidal conversion of thyroxine to 3,3',5'-triiodothyronine (reverse-T3) and to 3,5,3'-triiodothyronine (T3) in humans.
abstract: In order to estimate the relative magnitude of the two alternative pathways of monodeiodination of thyroxine (T4) in adult humans, the metabolic clearance rates (MCR) and production rates (PR) of 3,3',5'-triiodothyronine (reverse-T3,rT3) and of 3,5,3'-triiodothyronine (T3) were determined in six euthyroid control subjects (C) and in five hypothyroid patients (H) receiving L-T4 as replacement therapy (0.15-0.3 mg/day). MCR was computed by a non-compartmental method of analysis from the plasma disappearance of 125I rT3 and 131I T3 during 72 h following simultaneous injection of tracers. PR was calculated from MCR and the serum concentration of rT3 and T3, respectively, determined by radioimmunoassay. In the H subjects, rT3 MCR averaged 97.1 +/- 12.8 (SD) 1/day and rT3 PR, 34.3 +/- 12.8 microng/day; T3 MCR was 28.7 +/- 6.1 1/day and T3 PR, 20.3 +/- 6.6 microng/day (all corrected to 70 kg body weight). These results were not significantly different from those in the control group; rT3 MCR 104 +/- 24 1/day, rT3 PR 33.0 +/- 9.2 microng/day; T3 MCR 24.0 +/- 5.9, T3 PR 24.2 +/- 4.1. The proportionof total triiodothyronine (rT3 averaged 62% in H patients and was similar (57%) in the C group. The results obtained in the H subjects indicate that the production of rT3 is a major route of T4 metabolism, equal to or exceeding that of T3. From the close agreement between the mean values for rT3 PR in the C and H groups it is concluded that most, if not all of the rT3 produced in normal humans is derived by extrathyroidal conversion from T4.
mesh_terms: D000368:Aged; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000378:metabolism; D007536:Isomerism; D008297:Male; D008657:Metabolic Clearance Rate; D008875:Middle Aged; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*
chemical_list: D014284:Triiodothyronine; D013974:Thyroxine