Calorigenic Effects of Single Intravenous Doses

In 1950 and 1951 two unknown radioiodinebearing compounds were observed (1-3) on chromatograms of plasma obtained from animals treated with I'31. Gross and Pitt-Rivers (4) identified one of these as 3,5,3'-l-triiodothyronine, which they had previously isolated (5) from the plasma of euthyroid and hyperthyroid individuals treated with 131". They excluded the possibility of analytical or radiation artefact and considered 3,5,3'-l-triiodothyronine to be a normal constituent of the organic iodine moiety of plasma. Concomitantly, Roche, Lissitzky, and Michel (6) reported the conversion in vitro of diiodothyronine to triiodothyronine and crystallization of the latter compound. On the basis of its ability to prevent goiter in rats treated with thiouracil, Gross and Pitt-Rivers (7) found triiodothyronine three times as active as thyroxine. They also demonstrated its effectiveness (8) in raising the metabolic rate and correcting the symptoms of myxedema in two patients. In this respect they considered it to be at least as calorigenic as 1-thyroxine and probably more so, and suggested that it might prove to be the tissue form of the thyroid hormone. Since 3,5,3'-l-triiodothyronine is the only other amino acid to be found thus far which possesses hormonal properties comparable in intensity to those of thyroxine, it appeared desirable to make qualitative and quantitative comparison of the calorigenic properties of the two substances in human subjects.