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
The MHRA-agreed letter to healthcare professionals (May 2023) explains that biotin may interfere with thyroid immunoassays based on a biotin and streptavidin interaction, and that these methods are commonly used in clinical practice. Extra biotin disrupts that interaction. In a sandwich assay it lowers the signal, causing falsely low values; in a competitive assay it causes falsely high values. Which way a result is distorted therefore depends on the test design, and the letter warns this may lead to inappropriate patient management or misdiagnosis. The letter covers thyroid tests, but other immunoassay tests can be affected in the same way, so it is worth mentioning biotin whatever blood test you are having.
Practical advice
Tell the doctor or laboratory that you take biotin before a thyroid blood test, including any "hair, skin and nails" supplement or multivitamin, because many contain it. The letter says patients should be routinely asked about biotin use before thyroid tests, and that if a patient is taking biotin the laboratory should be told, because alternative assays might be available. It does not set a stopping time, so follow the advice of the laboratory that will run your test; a common precaution after high-dose supplements is to stop biotin for a few days beforehand. If thyroid results do not match how you feel, ask whether biotin could be the reason. Do not stop a prescribed medicine, including levothyroxine, without advice.