Evaluation of thyroxine monodeiodination to reverse triiodothyronine by tissue homogenates in vitro.

Reverse-triiodothyronine (rT3) generated from thyroxine (T4) in vitro at physiological pH (7.4) is very labile and rapidly undergoes further enzymic deiodination to diiodothyronine. The paper describes the procedure in which by different pH optima, T4 to rT3 conversion distinct from T4 to T3 monodeiodination can be traced at pH 9.0, in the presence of dithiothreitol (DTT) as a donor of sulfhydryl groups. The net in vitro production of rT3 from exogenous T4 was thus measured utilizing specific RIA for the hormone, in several pig tissue homogenates (liver, kidney, skeletal muscle, brain, heart). The thyroxine 5-deiodinase activity was found to be several times higher in the kidney and liver than in other tissues. The procedure can be used for observations of changes in T4 to rT3 conversion under various experimental conditions.