Background: Free T4 (FT4) and free T3 (FT3) immunoassays exhibit wide inter-assay variations. We therefore established control values with three different immunoassays and compared their clinical performances in thyroidal diseases and severe/acute non-thyroidal illnesses (NTI).
Methods: The UniCel DxI 800, Architect i2000, and Elecsys 2010 assays were used. FT4 and FT3 reference ranges were established in 68 controls, without conditions interfering with thyroid function, with normal TSH (0.35-3.02 mU/L) and negative anti-thyroid peroxidase antibodies. Free hormones were determined in 60 patients with thyroid diseases (TSH: < 0.001-31.5 mU/L) and 45 NTI patients (TSH: 0.10-4.72 mU/L). Control values were normalized as Z-scores; patients' results were expressed as Z-scores, using the control values of each assay. Pairwise comparisons of the Z-scores were performed by Deming's regression. Classification of patients' results based on 95% control values were evaluated by kappa agreement statistics.
Results: Control values for FT4 (pmol/L; geometrical means; 95% confidence intervals) were: 11.1 (7.6-16.1), 12.3 (9.1-16.6), 15.6 (11.4-21.4) and for FT3: 4.8 (4.0-5.7), 4.0 (3.0-5.3), 4.3 (3.1-5.09), with DxI 800, Architect, and Elecsys, respectively. Pairs of control Z-scores correlated significantly, but with different strengths (FT4: r = 0.915, 0.740, 0.770; FT3: r = 0.615; 0.589; 0.790, for DxI 800 vs. Elecsys; DxI 800 vs. Architect; Elecsys vs. Architect; p < 0.001); slopes and intercepts of paired controls were 1.00 and zero. In thyroid diseases, slopes of FT4 Z-scores among assays differed slightly from 1.00 (1.11, 0.88, 0.87 for DxI versus Elecsys, DxI 800 versus Architect, Elecsys versus Architect, respectively; p < 0.05); slopes of FT3 Z-scores were consistent with unity, except for DxI 800 versus Elecsys (0.88; p < 0.05). In NTI patients, regression slopes were consistent with unity (p < 0.05). The agreement statistics showed moderate to very good inter-assay concordances for thyroid and NTI patients' results.
Conclusions: FT4 and FT3 assays show moderate to very good agreement, in patients with thyroid diseases or NTI when compared pairwise to their control values. Slight quantitative differences between some pairs of assays are observed in thyroid diseases, after normalization as Z-scores.