Abstract: The methodology for determining FT3 in serum to estimate the patients with thyroidal and related diseases is established. In this new solid-phase antibody enzyme immunoassay, biotinylated T3 and HRP-streptavidin conjugate are used. The detection limit is low (0.90 pmol/L), the measurable range is 0.9 to 45.0 pmol/L, The intra- and inter-assay CVs are 3.1%~8.1% and (4.9)%~10.6%, respectively. FT3 values are obtained with this assay for 101 samples. The FT3 concentrations range is from 2.5~11.1 pmol/L in 46 euthyroid subjects, 9.7~37.3 pmol/L in 27 hyperthyroid patients, and 0.9~3.8 pmol/L in 20 hypothyroid patients and 3.4~8.5 pmol/L in 8 pregnalcy, respectively. Comparing with CIA and RIA, the clinical results correlated well. The method is suitable for clinical and research application.