3,3'-Diiodo-L-thyronine (3,3'-T2) is an endogenous metabolite of thyroid hormone. 3,3'-Diiodo-L-thyronine significantly enhances COX activity. 3,3'-Diiodo-L-thyronine (3,3'-T2; 1 μM; 30 min) significantly enhances COX activity[4]. The 3-iodine of 3,3'-T2 occupies the same site as the 3'-iodine of T4, and the metabolite 3'-iodine occupies the water site observed in the T4 complex. The binding affinity of 3,3'-T2, which is 100-fold lower than that of T4, reflects the lack of the second pair of iodine atoms interacting in the channel[5]. In hypothyroid rats, we determined the effects of administration of different doses of 3,3',5-triiodo-L-thyronine (T3), 3,3'-diiodo-L-thyronine (3,3'-T2) and 3,5-diiodo-L-thyronine (3,5-T2) ("T2 isomers' refers specifically to these latter two isomers throughout this paper) on resting metabolism (RM) and on the oxidative capacity (measured as cytochrome oxidase activity) of tissues that are metabolically very active. The T2 isomers induced a dose-dependent calorigenic effect when injected I.P. into hypothyroid rats. The effect of T2 isomers on RM was paralleled by an increase in the oxidative capacity of tissues that are metabolically very active (liver, skeletal muscle, brown adipose tissue (BAT) and heart). The increases were between 33% (liver + 3,3'-T2) and 63% (muscle + 3,3'-T2) [6].
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