BACKGROUND Since the introduction of the first radioimmunoassay, several improvements have been made in the design of immunoassays such as method of antibody production, labeling, automation and detection technology. We performed an analytical evaluation of the new electrochemiluminescent immunoassay (ECLIA) for serum TSH, FT4 and T3 in the Elecsys 2010 immunoassay system and compared the results of this method with those of radioimmunoassay ( RIA) [immunoradiometric (IRMA) for TSH] and Elisa. METHODS Fasting serum from 112 hypo, hyper and euthyroid patients were used to evaluate the minimum detectable concentration, intra- and inter-assay precisions for TSH, FT4, T3, linearity for TSH assay and method comparison study. RESULTS Within the analytical range tested, intra-assay coefficient of variation was < 2.3% for TSH, 2.3% for FT4 and 7.8% for T3. The inter-assay coefficient of variation was < 2.9% for TSH, 2.5% for FT4 and 12.3% for T3. The measurement of diluted sera indicated a desirable percentage of recovery for TSH. No correlation was found between Elecsys 2010 and Elisa /IRMA for TSH. The comparison of results of the Elecsys ECLIA assay with those of Elisa and RIA for T4 were: T4 (ECLIA) = -0.612+0.999, T4 (Elisa, r= 0.88) and T4 (ECLIA)=0.642+0.942 T4 (RIA, r=0.957), while ECLIA assay with Elisa and RIA for T3 were: T3 (ECLIA)= 0.242+0.908 T3 (Elisa, r=0.8) and T3 (ECLIA) = -0.029+1.01 T3 (RIA, r=0.957). CONCLUSION The results show that Elecsys 2010 is an automated reliable, efficient and technically excellent instrument to use in the measurement of serum TSH, T4 and T3.