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Research Article Free access | 10.1172/JCI106596

The extrathyroidal conversion rate of thyroxine to triiodothyronine in normal man

Constance S. Pittman, Joseph B. Chambers Jr., and Virginia H. Read

Department of Medicine, University of Alabama Medical Center, Birmingham, Alabama 35233

Find articles by Pittman, C. in: PubMed | Google Scholar

Department of Medicine, University of Alabama Medical Center, Birmingham, Alabama 35233

Find articles by Chambers, J. in: PubMed | Google Scholar

Department of Medicine, University of Alabama Medical Center, Birmingham, Alabama 35233

Find articles by Read, V. in: PubMed | Google Scholar

Published June 1, 1971 - More info

Published in Volume 50, Issue 6 on June 1, 1971
J Clin Invest. 1971;50(6):1187–1196. https://doi.org/10.1172/JCI106596.
© 1971 The American Society for Clinical Investigation
Published June 1, 1971 - Version history
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Abstract

Eight normal subjects were administered tracer amounts of a 14C-labeled thyroxine, L-[tyrosyl-14C] T4, by multiple injections. Then serial blood samples were collected for isolation of the thyroxine, triiodothyronine, and tetraiodothyroacetic acid fractions by a combination of column and paper chromatographies. The chromatographic artifacts were corrected by adding to the sera a purified 3H-labeled thyroxine, D,L-[α,β-3H] T4 immediately after the separation of sera from blood. 1-2% of the serum 14C radioactivity was observed in the triiodothyronine fraction and 2-4% of the serum 14C radioactivity was observed in the tetraiodothyroacetic acid fraction. Complete kinetic studies of thyroxine and triiodothyronine were compared in the same individual in four of the subjects. The extrathyroidal conversion rates of thyroxine to triiodothyronine were calculated from data obtained during both the injection and the postinjection periods as functions of the 14C-labeled thyroxine and triiodothyronine remaining in the body at time t and their fractional turnover rates. The average daily rate of the extrathyroidal conversion of thyroxine to triiodothyronine was 4% of the extrathyroidal thyroxine pool or 33% of the total thyroxine production. The amount of triiodothyronine generated by this pathway (22 μg/day) was found to contribute 31% of the extrathyroidal triiodothyronine pool or 41% of the daily triiodothyronine production. This pathway is a major source of triiodothyronine production. The extrathyroidal conversions of thyroxine to triiodothyronine and tetraiodothyroacetic acid are major metabolic pathways of thyroxine in normal man.

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