Spuriously Elevated Free Thyroxine Due to Fibrin Interference in a Patient Receiving Apixaban: A Case Report and Literature Review
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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BibTeX
@article{suzuki2026spuriously,
title = {{Spuriously Elevated Free Thyroxine Due to Fibrin Interference in a Patient Receiving Apixaban: A Case Report and Literature Review}},
author = {Takahiro Suzuki and Kenji Ohba and Akiko Agata and Shiori Takebayashi and Keita Yamashita and Mitsuaki Tokumaru and Hiroshi Ukigai and Ryosuke Takeuchi and Takayuki Iwaki and Hirotomo Saitsu and Masato Maekawa and Yuko Suzuki},
journal = {Internal Medicine},
year = {2026},
month = aug,
doi = {10.2169/internalmedicine.6473-25},
url = {https://www.jstage.jst.go.jp/article/internalmedicine/65/15/65_6473-25/_article},
note = {CC BY-NC-ND},
}
RIS
TY - JOUR
TI - Spuriously Elevated Free Thyroxine Due to Fibrin Interference in a Patient Receiving Apixaban: A Case Report and Literature Review
AU - Takahiro Suzuki
AU - Kenji Ohba
AU - Akiko Agata
AU - Shiori Takebayashi
AU - Keita Yamashita
AU - Mitsuaki Tokumaru
AU - Hiroshi Ukigai
AU - Ryosuke Takeuchi
AU - Takayuki Iwaki
AU - Hirotomo Saitsu
AU - Masato Maekawa
AU - Yuko Suzuki
PY - 2026
DA - 2026/08/01
JO - Internal Medicine
DO - 10.2169/internalmedicine.6473-25
UR - https://www.jstage.jst.go.jp/article/internalmedicine/65/15/65_6473-25/_article
AB - 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.
SN - 1349-7235
SN - 0918-2918
ER -