In this method, Sephadex is used to assess the percentage of free thyroxine in undiluted sera. We demonstrate that if temperature and amount of Sephadex are kept constant, reproducibility is excellent. When Sephadex is exposed to serum, equilibration between thyroxine and thyroxine-binding components of the system is instantaneous. Because it is so rapid, it is not possible to measure the true physiological fraction of free thyroxine in serum with any free-thyroxine-binding resin, but the method does give an accurate measure of fluctuations in percentage of free thyroxine, as indicated by the high correlation with values for percentage of free thyroxine as determined by equilibrium dialysis and by the appropriate response to additions of exogenous thyroxine and dilution of the serum.
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Levinson et al. (1974) studied this question.
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