Metabolism of the thyroid hormone in the rat as shown by physiological doses of labeled thyroxine.

Since thyroxine, uncombined in peptide linkage, has been detected by isotope dilution and paper chromatography in thyroid tissue (1, 2) and in plasma (l-3), it is likely that this substance is the hormonal secretion of the thyroid gland. Previously, the fate of labeled thyroxine has been determined after the injection of large amounts (4-6), considerably in excess of the few micrograms which constitute the daily thyroxine secretion of the rat thyroid (7). In the present work, methods were devised by which it was possible to prepare 0.007 y of labeled thyroxine containing enough radioactivity to be easily traced in the body of the rat. The distribution of thyroxine thus obtained was then found to be comparable to that of radioiodide (NaI131) in rats sacrificed long enough after injection to insure the conversion of the radioiodide to thyroid hormone.