1. Department of Medicine, Columbia University College of Physicians and Surgeons, and Presbyterian Hospital, New York.

Abstract

It has been shown by a number of investigators that L-thyroxine is more active in vivo than is D-thyroxine¹. In most in vitro systems, however, the D-and L-isomers are equally effective. The equal activity of D- and L-thyroxine in altering the morphology of mitochondria and in 'uncoupling' oxidative phosphorylation are examples of this^2,3. Previous experiments⁴ in this laboratory have compared the distribution and excretion of labelled D- and L-thyroxine labelled with iodine-131 following a single injection of a tracer quantity of each compound into the tail vein of the rat. Initially, D-thyroxine was concentrated to a greater extent in the liver and kidney than was L-thyroxine, but after 24 hr. the concentrations were approximately equal. However, the concentration of D-thyroxine in tissues such as muscle, brain and skin was much less than the concentration of L-thyroxine. This difference in distribution to the peripheral tissues was of such magnitude as to suggest a possible basis for the different activities of the two isomers in vivo. Others have shown^5,6 that the oxygen consumption of certain tissues (for example, liver, kidney and muscle) from animals injected with DL-thyroxine is significantly greater than normal, whereas that of other tissues (for example, brain) is not increased. The current investigation was undertaken to determine whether the differences demonstrated in the distribution of the two isomers could be correlated with a difference in their ability to stimulate the oxygen consumption of the several tissues.