BACKGROUND: The analytical performance of the Vitros ECi(R) Immunodiagnostic System on the thyroid stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4) and estradiol (E2) assays, which are based on electrochemical luminescence to replace the radioimmunoassay was evaluated. METHODS: The precision as measured by the NCCLS protocol and a comparison of the method were done for each TSH, FT3, FT4, and E2 assay. The functional sensitivity and linearity tests were performed for both TSH and E2. The free hormone validity test was performed for both the FT3 and FT4. RESULTS: All four analytes showed an acceptable precision. The functional sensitivities of TSH and E2 were 0.007 mIU/L, and 87 pmol/L, respectively. TSH and E2 showed excellent linearity up to 78 mIU/mL, and up to 7,700 pmol/L, respectively. The free hormone validity test showed acceptable results demonstrating accurate free hormone determination. The E2 showed a significant proportion-al bias requiring an adjustment of the reference range, However, the other analytes showed good agreement with a slight proportional bias. CONCLUSIONS: The TSH, FT3, FT4, and E2 assay by Vitros ECi(R) exhibited excellent performance overcoming the drawbacks of a conventional radioimmunoassay.