Spuriously Elevated Free Thyroxine Due to Fibrin Interference in a Patient Receiving Apixaban: A Case Report and Literature Review

Suzuki T, Ohba K, Agata A, Takebayashi S, Yamashita K, Tokumaru M, Ukigai H, Takeuchi R et al.

Internal medicine (Tokyo, Japan), 2026

1 citation

Abstract

Fibrin interference can cause spurious immunoassay results, particularly in patients with coagulopathy or those receiving anticoagulant therapy. We report the case of an asymptomatic 79-year-old woman treated with apixaban, who presented spuriously elevated free thyroxine and free triiodothyronine levels, mimicking a syndrome of inappropriate secretion of thyroid-stimulating hormone. Plasma measurements revealed values within the reference range, which confirmed fibrin interference. To our knowledge, this is the first case to demonstrate fibrin interference in thyroid function testing during direct oral anticoagulant therapy. The relevant literature suggests a greater susceptibility to specific free thyroxine assay platforms. Clinicians should recognize this phenomenon to avoid unnecessary investigation of apparent thyroid dysfunction.

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