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Dear Editor, Interactions of thyroid hormone autoantibodies (THAAs) with thyroxine (T4) or triiodothyronine (T3) are less common than interactions of antibodies to thyroglobulin, microsomal thyroid peroxidase, and thyroid-stimulating hormone (TSH) receptor. Nevertheless, these THAAs have been reported to cause spurious thyroid function test (TFT) results, indicating that TFT results do not always accurately reflect the clinical status of the patient and are not necessarily consistent [1, 2]. The susceptibility to THAA interference depends particularly on whether the patient serum components are exposed to the tracers used (e.g., one-step vs two-step immunoassay, equilibrium dialysis, or ultrafiltration methods) and not the label used in the tracers. In two-step methods, all serum components are removed by a washing step before addition of the tracer [1]. Therefore, automated one-step immunoassays are known to be more susceptible to THAA interference [2]. We report the first case of the spurious elevation of free T4 in a Korean patient caused by circulating THAA. A 40-yr-old female presented with fatigue and edema spanning a three-month period. She had taken methimazole owing to elevated free T4 and TSH levels detected on a TFT performed in another hospital. For proper management, she was transferred to our hospital. The endocrinologist ordered a TFT from the Departments of Laboratory Medicine and Nuclear Medicine. Total T3, total T4, free T4, and TSH were measured with a one-step chemiluminescent immunoassay (CLIA) on the ADVIA Centaur XP system (Siemens Healthcare, Munich, Germany) and with an RIA. The serum concentrations of total T3 and free T4 were measured by using a total T3 RIA kit (Immunotech Inc., Praha, Czech Republic) and a free T4 RIA kit (Immunotech), respectively. Free T4 was quantitated with a two-step CLIA on an Architect i2000 system (Abbott Diagnostics, Santa Clara, CA, USA). The TFT results obtained from the Department of Laboratory Medicine were comparable with the results from the outside hospital (Table 1). By contrast, the results from the Department of Nu-

Thyroxine (T4) Autoantibody Interference of Free T4 Concentration Measurement in a Patient With Hashimoto’s Thyroiditis