The clinical and physiological comparisons of the isomers of thyroxine by a number of investigators are well summarized by Tabachnik et al. in a recent publication.1 Some excellent animal measurements of oxygen consumption suggest complete lack of potency of the dextro- isomer,2 whereas other studies,3 with use of a different phenomenon, demonstrate physiological activity. Most of these studies were done with racemic mixtures of the two isomers, as compared with the results of the administration of the levo- isomer alone. A generous provision of unlimited quantities of sodium D-thyroxine, suitable for clinical studies, has permitted an extended series of human observations.* Amounts of the sodium D-thyroxine sufficient to administer 100 times the usual levo- isomer dosage have made possible hitherto unavailable comparisons. The present studies, however, were done at the dose levels regularly used clinically in the treatment of hypothyroidism with L-thyroxine.
The chemical characteristics of the