Effect of thyroid-suppressive doses of triiodothyronine on thyroxine turnover and on the free thyroxine fraction

George C. Schussler; Vernon K. Vance

doi:10.1172/JCI105767

¹Department of Medicine, State University of New York at Buffalo, Edward J. Meyer Memorial Hospital and The Buffalo General Hospital, Buffalo, New York 14215

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¹Department of Medicine, State University of New York at Buffalo, Edward J. Meyer Memorial Hospital and The Buffalo General Hospital, Buffalo, New York 14215

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Published in Volume 47, Issue 4 on April 1, 1968
J Clin Invest. 1968;47(4):720–728. https://doi.org/10.1172/JCI105767.
© 1968 The American Society for Clinical Investigation

Published April 1, 1968 - Version history

The relationship between free thyroxine concentration and thyroxine turnover was studied during thyroid suppression with triiodothyronine. Although there was some increase in the proportion of serum thyroxine bound to thyroxine-binding globulin, the ratio of ultrafilterable to protein-bound hormone was not significantly affected. The fractional disappearance rate of thyroxine increased from an average control value of 11.47%/day to 14.72%/day. Because of contraction of the thyroxine distribution space the clearance of thyroxine was less markedly affected, increasing from 1.37 to 1.56 liters/day. Since the ratio of thyroxine turnover to free thyroxine concentration, i.e., the free thyroxine clearance, increased proportionately (4.79-5.55 liters × 10³/day) we conclude that triiodothyronine stimulates thyroxine clearance by a mechanism that is independent of effects on free thyroxine concentration.