AVOID taking high-dose biotin within at least 48 to 72 hours of a thyroid function test (TSH, free T4, free T3) because biotin can produce falsely high or falsely low results on most immunoassay-based UK lab platforms.
Mechanism
Most UK lab platforms (Roche, Abbott, Siemens) use streptavidin-biotin chemistry to bind reagents to antibodies in the immunoassay. High-dose oral biotin saturates these binding sites and produces analytically unreliable results: the artefact direction depends on the platform (competitive vs sandwich assay) and on which thyroid parameter is being measured. The same artefact affects troponin, hormones, vitamin D and many other immunoassay biomarkers.
Practical advice
Stop high-dose biotin (1 mg or more daily; many "hair, skin and nails" supplements contain 5 to 10 mg) for at least 48 to 72 hours before any blood draw that may include thyroid, troponin, hormone or vitamin D testing. If you cannot stop biotin (e.g. clinical reason), tell the requesting clinician and the lab at the time of the draw so they can use a biotin-tolerant assay or note the limitation on interpretation.