Multicentre comparison of free thyroid hormones immunoassays: the Immunocheck study.

Background: In order to evaluate whether there is still present a very large variability in measured values and analytical performance among different free thyroid (FT) hormone immunoassays, we considered the results derived from an External Quality Assessment (EQA) scheme, named Immunocheck.

Methods: The EQA Immonocheck study enrolled about 1000 participant laboratories, which measured the 54 quality control samples distributed in the three annual cycles (2007, 2008 and 2009). Participant laboratories produced a total of 30,476 results for FT3 and 31,351 for FT4, respectively.