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

Objectives: This study aimed at evaluating Elecsys® 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 (NTIm, n = 31), or intensive care units (NTIc, 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, NTIm, NTIc, and amiodarone groups. Use of age-related reference ranges improved the specificity markedly in amiodarone patients and to a lesser extent in NTIm and NTIc 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.