Measurement of free triiodothyronine in intensive care patients--comparison of two routine methods.

One hundred sera from intensive care patients, and 93 sera from endocrinological outpatients were used for a comparison between two automated assays for free triiodothyronine (Enzymun Test, and Elecsys 2010, both Boehringer Mannheim, Mannheim, Germany). In outpatients a good correlation between both methods was found (r = 0.932). In contrast, comparability between the two assays was poor in intensive care patients (r = 0.75, after exclusion of two outliers); significantly more values in the hypothyroid range were found with the Elecsys 2010 assay (n = 83, compared with n = 33 with the Enzymun Test; chi 2 test p = 0.001). We conclude that routine measurement of free triiodothyronine which has the theoretical advantage of quantifying the biologically active fraction of thyroid hormones may have methodological limitations in severely ill patients.