[Determination of free thyroxine by adsorption on Sephadex gel]

The authors describe a simplified method for the measurement of free thyroxine based on the affinity of Sephadex gel for thyroxine. Radiolabeled thyroxine added to diluted serum is adsorbed by the gel proportionally to the free thyroxine concentration of the sample. The concentration of binding proteins present in the sample does not affect the adsorption of free thyroxine on the gel. The adsorbed thyroxine is separated by centrifugation and filtration using a rigid filter. Calibration is made against the equilibrium dialysis method. The intra (CV of 4.9 p. cent, 6.1 p. cent, 10.2 p. cent for concentrations of free thyroxine of 14.1, 27.1 and 67.6 pmol/l respectively) and interassay (CV of 5.3 p. cent, 8.2 p. cent, 9.2 p. cent at 9.4, 23.0 and 83.1 pmol/l respectively) imprecisions are comparable to equilibrium dialysis. The results obtained with this method show a good correlation with those of a two-step radioimmunoassay for patients without non-thyroidal diseases or pregnancy (r2 = 0.85, n = 50). The reference range (mean +/- two standard deviations) determined in 108 euthyroid adults is 10.3 to 21.9 pmol/l. Results obtained for hypothyroid patients (7.7 +/- 2.3 pmol/l, mean +/- one standard deviation, n = 26), hyperthyroid patients (47.4 +/- 22.9 pmol/l, n = 32), patients with non-thyroidal illnesses (18.7 +/- 4.9 pmol/l, n = 35) and pregnant women (10.8 +/- 1.7 pmol/l, n = 20) are in good agreement with those reported using equilibrium dialysis or ultrafiltration. Our method offers an alternative to more tedious techniques of free thyroxine determination in the hospitalized population for which the use of analog methods has been criticized.