Free thyroxine in human serum: simplified measurement with the aid of magnesium precipitation.

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Abstract
A simplified method was developed for the measurement of unbound thyroxine in serum. Serum was dialyzed after addition of tracer 131I-labeled thyroxine. The dialyzate was enriched with nonradio-active thyroxine followed by precipitation with Mg. The washed precipitate contained virtually all the dialyzed tracer thyroxine freed of contaminating iodide. The free thyroxine fraction represented the thyroxine radioactivity in the dialyzate divided by thyroxine radioactivity in the original undialyzed serum. The mean free thyroxine was 0.046% of the total thyroxine of normal serum, corresponding to a concentration of 5.4 x 10-11 mole/1. The free thyroxine values, expressed as % of the total thyroxine content of the sera, were (mean [plus or minus] standard deviation): thyrotoxicosis 0.104[plus or minus]0.026, euthyroidism 0.046[plus or minus]0.005, hypothyroidism 0.028[plus or minus]0.008, pregnancy 0.026[plus or minus]0.004, and euthyroid "sick" group 0.088[plus or minus]0.036. The product of the % free thyroxine and the protein-bound I gave a value for free thyroxine I concentration that markedly deviated from normal in thyrotoxicosis and hypothyroidism. This value was probably more closely correlated with thyroid function than the total hormone concentration.