Evaluation of Elecsys® free triiodothyronine assay:: Relevance of age-related reference ranges

Objectives: This study aimed at evaluating Elecsys(R) free triiodothyronine (FT3) assay performed on an Elecsys 2010 system, while paying special attention to age relationship in euthyroid subjects. Design and Methods: FT3 levels were measured in 149 euthyroid control subjects aged between 2 and 92 years old, 33 hyperthyroid and particular euthyroid patients: female in the last 3 months of pregnancy (n = 30), nonthyroidal ill hospitalized in medical (NTlm, n = 31), or intensive care units (NTlc, n = 31) and amiodarone-treated (n = 27). Results: FT3 was inversely related to age in controls (r = -0.67). Three reference ranges were used: below 20 years 4.5-9.0 pmol/L, between 20 and 60 years 3.9-7.2, and over 60 years 2.4-6.5. Compared to age-matched controls, FT3 decreased in pregnancy, NTlm, NTlc, and amiodarone groups. Use of age-related reference ranges improved the specificity markedly in amiodarone patients and to a lesser extent, in NTlm and NTlc patients. Conclusions: The reliability of the Elecsys FT3 assay was found to be satisfactory for clinical use, when the age of patients was taken into account. Copyright (C) 1998 The Canadian Society of Clinical Chemists.