Interference in thyroid function tests using the electrochemiluminescence immunoassay

Objective: Electrochemiluminescence immunoassays (ECLIAs) are widely used for thyroid function testing but may be affected by rare assay interferences that can lead to misdiagnosis. We aimed to investigate the frequency and types of assay interferences in thyrotropin (TSH), free thyroxine (FT4), and free triiodothyronine (FT3) measurements using the Roche ECLIA. Methods: Between 2019 and 2023, 124,615 patients underwent thyroid function testing. Assay interference was suspected in 283 cases based on clinical-laboratory mismatch, thyroid hormone imbalance, or discrepancies between assay methods. Further validation assessed potential interferences. Results: Of the total tests (TSH: 124,609; FT4: 116,012; and FT3: 83,044), 71 interference cases were confirmed (TSH: 11 (0.009%), FT4: 6 (0.005%), and FT3: 53 (0.064%)). FT3 interferences were mainly due to idiotype and anti-streptavidin antibodies. Macro-TSH was detected in six cases and often led to unnecessary levothyroxine treatment. Furthermore, most interferences were detected by recognizing hormonal imbalances rather than clinical symptoms. Conclusion: FT3 tests are most prone to assay interference. Awareness of inter ference patterns and close collaboration between physicians and laboratory staff are essential to prevent diagnostic errors.