Enzyme immunoassay of free thyroxin in serum

We describe a double-antibody enzyme immunoassay for free thyroxin (FT4) in serum with use of beta-D-galactosidase conjugated to thyroxin. The method is uninfluenced by thyroxin-binding globulin or albumin. Values for FT4 so determined correlated well with those determined by radioimmunoassay (r = 0.98) and equilibrium dialysis (r = 0.89). The mean variability (CV) within and between assays was 7.4% and 7.6%, respectively. The measurable range of FT4 in serum was 2.8 to 109 ng/L. The FT4 concentrations in serum as determined by this method were 8.4 to 15.5 ng/L for 26 normal adult subjects; 26 to greater than 109 ng/L for 10 patients with hyperthyroidism; less than 2.8 to 8.0 ng/L for seven patients with hypothyroidism; 7.3 to 15.8 ng/L for eight pregnant women; and 12.2 and 13.5 ng/L for two patients with low concentrations of thyroxin-binding globulin.