Stereospecific determination and in vivo monodeiodination of thyroxine enantiomers in euthyroid man

To compare in man the absorption, serum disappearance, and peripheral monodeiodination of the thyroxine enantiomers, we studied six euthyroid subjects who, on separate occasions, orally ingested 3 mg of either dextrothyroxine (DT₄) or levothyroxine (LT₄). We measured the serum concentrations of total T₄ (TT₄), total T₃, and reverse T₃ (rT₃) by nonstereospecific radioimmunoassay and we determined serum DT₄ by stereospecific chromatography. Mean serum TT₄ levels from 4 hours were significantly greater after LT₄ administration. After DT₄ administration, stereospecific analysis of serum revealed two T₄ peaks that persisted from 2 to 48 hours. The mean serum LT₄ leved did not significantly change during the 48 hours after DT₄ administration. Increases in serum T₃ and rT₃ were seen from 2 hours after administration of either enantiomer. From 12 hours the levels of both triiodothyronines after LT₄ were significantly higher than after DT₄. In this short term study we found no evidence that in man DT₄ is converted to LT₄, nor is it preferentially deiodinated to rT₃. The greater and more persistent increases in serum T₄ and T₃ observed after LT₄ probably contribute to the known higher bioactivity of that enantiomer.