Physiological disposition of D- and l-triiodothyronine in rats

¹ Department of Medicine, Columbia University College of Physicians and Surgeons; and the Presbyterian Hospital, New York City

The physiological disposition in the rat of d- and l-triiodothyronine has been compared. Following intravenous injection, the two isomers were removed from the circulation at similar rates but were taken up by different tissues. d-Triiodothyronine was concentrated in the liver and kidney to a greater extent than the l-isomer. On the other hand, higher concentrations of l-triiodothyronine were found in all other tissues, especially brain and muscle. With the d-isomer, a greater fraction of the radioactivity was excreted in the urine, presumably as the result of a more rapid deiodination. As in previous studies with thyroid hormones, the disposition observed seemed to correlate well with the known functions of the two isomers in vivo. The results emphasize how meaningless is the calculation of a "space" or "volume of distribution" of any particular compound on the basis of blood levels alone.