Determination of free thyroxine in serum by radioimmunoassay.

We describe a sensitive and specific radioimmunoassay for free thyroxine, directly determined in the diffusate obtained from equilibrium dialysis of serum. The intra- and interassay precisions were 7.5 +/- 0.27 and 9.1 +/- 0.58 ng/liter (mean +/- SD), respectively. The free thyroxine concentrations of normal persons and patients with thyroid disease were as follows: normal, 7.6 +/- 1.7; euthyroid patients receiving phenytoin, 6.9 +/- 1.4; hypothyroidism, 2.9 +/- 1.3; and hyperthyroidism, 32.3 +/- 12.3 ng/liter (mean +/- SD).