To the Editor,In 2014, in this same journal, Zaninotto et al.  indicated an interference with the newly modified ruthenium label utilized in Roche free thyroxine assays (fT4 II generation) by an unidentified factor. It was the first available description of an interference other than the well-known anti-ruthenium antibodies in fT4 assays.In the following case, we will describe the presence of a particular interfering factor with Roche fT4 assays that may be unexpectedly enhanced via prewashing procedures.A 42-year-old woman presented to the Endocrine Unit of the University Clinic Hospital of Valencia after a thyroid nodule was found in primary care with no other signs or symptoms related to thyroid illness. She was not in thyroid treatment or under relevant medication that could alter thyroxine-binding globulin (TBG), albumin, and prealbumin levels.We measured a first sample (serum A), obtaining a very high free thyroxine (fT4), 63.7 pmol/L [reference range (RR), 12.0–22.0], with normal thyroid-stimulating hormone (TSH), 0.8 mU/L (RR, 0.27–4.20), and total triiodothyronine (tT3), 3.1 nmol/L (RR, 1.2–3.1) (Table 1). The biochemist and the endocrinologist doubted the validity of this result, given that the patient did not appear thyrotoxic. To rule out any analytical interference [2, 3], we decided first, to
Clinical Chemistry and Laboratory Medicine (CCLM) – de Gruyter
Published: Jul 26, 2017
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