Studies of the Peripheral Metabolism of Triiodothyronine and Reverse Triiodothyronine in the Rat: A Comparison of Extraction and Chromatographic Methods of Analysis*

Abstract

Studies were performed of the peripheral metabolism of radioiodine-labeled T3 and rT3 in the rat. Two methods of analysis were employed to determine the residual concentrations of labeled hormones after their single iv injection: trichloroacetic acid-ethanol extraction, as employed by others, and a recently described technique for the chromatography of plasma on columns of Sephadex G-25. In the case of both T3 and rT3, Sephadex chromatography regularly revealed the appearance in plasma of labeled iodoprotein, radioiodide, and a peak of unidentified radioiodinated materials that eluted just before T3 (pre-T3). Radioiodine in the pre- T3 zone, whether generated from T3 or rT3, proved to be almost completely TCA-precipitable and ethanolextractable. Consequently, for both T3 and rT3, plasma disappearance curves derived by the precipitation-extraction technique were at all time points higher than were curves representing chromatographically isolated administered hormone. In the case of rT3, and to a lesser ext...