Harmonization of thyroid function test measurements across multiple immunoassay platforms for a common reference...

Reference intervals for thyroid function tests vary across immunoassay platforms, leading to inconsistent clinical interpretations. This study aimed to establish a harmonized common reference interval through recalibration and evaluate its clinical utility.

Leftover serum samples from adults undergoing health examinations (n = 283 for thyrotropin [formerly thyroid-stimulating hormone], n = 265 for triiodothyronine [T3], and n = 283 for free thyroxine [FT4]) were analyzed on Abbott, Beckman Coulter, and Roche platforms. Percentile transformation with regression-based recalibration was applied to establish method-specific reference intervals, which were integrated into a common reference interval. Agreement was evaluated using coefficients of variation (s/x̄) × 100 (CVs), width ratios, overlap percentages, and limit-difference percentages.

Recalibration markedly reduced variability for thyrotropin (midpoint CV, 10.7% → 2.9%) and FT4 (22.7% → 6.1%). In contrast, T3 demonstrated limited improvement (midpoint CV, 5.1% → 9.3%; overlap, 73%-78%) and a large Abbott lower-limit discrepancy (ΔL% = 21.1%). Manufacturer-provided reference intervals demonstrated poor agreement (as low as 88% for thyrotropin and 91% for T3), whereas recalibrated and common reference intervals achieved 97% to 99% concordance.

Percentile transformation with regression-based recalibration enables practical and cost-effective harmonization of thyrotropin and FT4 reference intervals. Although T3 harmonization remained limited, this approach demonstrates the feasibility and clinical value of harmonized common reference intervals for improving interpretive consistency in thyroid function tests.