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 and LT4 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 level 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.