Measurement method for radioactive thyroxine, triiodothyronine, iodide, and iodoprotein in samples with low activity

A method is described that incorporates resin extraction and thin layer chromatography to isolate and separate radioiodinated thyroxine (T4), triiodothyronine (T3), iodoprotein, and iodide in samples of human plasma up to 3 ml. Tracer studies using this method showed that reverse T3 and 3',5' diiodothyronine (T2), as well as T4, were detected in the "T4 fraction," and that 3-3' T2 and 3' monoiodothyronine, as well as T3, were detected in the "T3 fraction." Monoiodotyrosine and diiodotyrosine (DIT) migrated more slowly than did T4 on the chromatogram, and a large amount of DIT was in the unextracted "iodoprotein fraction." Kinetic studies in 14 normal subjects given intravenous commercial [125I]T3 (T3*) and [131I]T4 (T4*), confirmed the quantitative importance of an iodoprotein in later samples after T3* administration, and its presence after T4*. T4* contamination of commercial T3* also became quantitatively important. On the other hand, despite confirmation of in vivo conversion of T4* to T3*, T3* contributed little quantitatively to the total concentration of radioactivity present even late after T4* injection, due to the more rapid turnover and greater distribution volume of T3*.