2.2. Radioimmunoassay
Serum concentrations of free T3 (fT3), total T3 (tT3) and total T4 (tT4) were measured by radioimmunoassay (RIA), using commercial kits (North Institute of Biotechnology, Beijing, China). The detection limit for fT3, tT3 and tT4 were 0.4 pg/mL, 0.2 ng/mL and 5 ng/mL, respectively. The intra- and inter-assay coefficients of variation were 5% and 10%, respectively, for all the three measurements. The commercial RIA kits were validated for measuring chicken samples previously (Li et al., 2007).