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Measurement Method for Radioactive Thyroxine, Triiodothyronine, Iodide, and Iodoprotein in Samples with Low Activity

VA Medical Centers, Buffalo, New York, and Washington, DC
VA Central Office, Washington, DC
State University of New York at Buffalo, New York

Correspondence: For reprints contact: Marguerite T. Hays, M.D. (15), Research and Development, VA Central Office, 810 Vermont Ave., NW, Washington, DC 20420.

ABSTRACT

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*.