Background: Thyroid function tests (TFT) produce varying results depending on the method, complicating standardization due to the lack of reference materials and methods. This study aims to harmonize TFT methods by deriving a recalibration equation using percentile transformation.
Methods: Data from the Korean Association of External Quality Assessment Service (2017–2020) were analyzed, focusing on the three most used automated immunoassay analyzers. Outliers were excluded, and data were transformed into percentiles. A recalibration equation was derived through regression analysis, and the harmonization of results before and after recalibration was evaluated. Clinical sample measurements using the three methods and their reference intervals were applied to the recalibration equation.
Results: Before recalibration, significant differences between methods were observed: 1.08 to 2.67 μIU/mL thyroid-stimulating hormone (TSH), 0.17 to 0.49 ng/mL triiodothyronine (T3), and 0.08 to 0.63 ng/dL free thyroxine (FT4). After recalibration, these differences were significantly reduced to 0.09 to 0.23 μIU/mL TSH, 0.002 to 0.006 ng/mL T3, and −0.01 to 0.02 ng/dL FT4. The distribution of clinical sample results remained consistent based on the reference interval before and after recalibration. However, differences persisted when applying clinical sample results to the recalibration equation. The difference in the TSH reference interval increased after recalibration, whereas the FT4 reference interval aligned more closely between methods.
Conclusions: Future studies should include multiple centers, sufficient clinical samples with various result levels, and multiple reagent lots. These studies should derive recalibration equations, and results from healthy individuals using various methods should be applied to establish a common reference interval.

Keywords: Harmonization, Standardization, Thyroid function tests, External quality assessment, Percentile transformation, Recalibration