Abstract
A 41-year-old woman on a 10 mg daily biotin supplement presented with palpitations and was found to have a thyroid profile
consistent with florid thyrotoxicosis on a streptavidin-biotin immunoassay platform: suppressed TSH, elevated free T4 and free
T3. She had no clinical signs of thyrotoxicosis and a normal thyroid ultrasound. Repeat testing after a 72-hour biotin washout, and
confirmatory testing on a non-biotin-based platform, showed a normal thyroid profile throughout. We describe the assay
chemistry that makes competitive and sandwich immunoassays using streptavidin-biotin capture systematically vulnerable to
high-dose biotin, why the direction of the artefact differs between assay formats, and a practical laboratory pathway for flagging
the result before it reaches a clinician rather than after a patient has been referred for unnecessary treatment.
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