THE RELATIVE THYROIDAL POTENCY OF 1- AND d,1-THYROXINE1

ALTHOUGH it was established by Harington and Salter (1930) that thyroxine naturally occurs in the thyroid in the optically active levo form, there is still considerable uncertainty as to the relative potency of pure 1-thyroxine and the racemic thyroxine that until recently has been the only form readily available for animal experimentation and clinical use. Using compounds prepared by Harington (1928) Gaddum reported that, as tested by the stimulation of tadpole metamorphosis (1927) and rat metabolism (1929-30), 1-thyroxine showed about three times the potency of the d- form. Upon testing the same compounds in human myxedema, Salter, Lerman and Means (1935) found no difference in activity. On the other hand Foster, Palmer and Leland (1936) reported that 1-thyroxine obtained by the enzymatic hydrolysis of thyroid substance exerted twice the effect of a racemic mixture as judged by its calorigenic effect in guinea pigs, and thus inferred that the d- compound is devoid of activity.