Taking high-dose supplements of the B vitamin biotin can throw off the results of some common diagnostic blood tests. Many people take biotin in hopes of stimulating growth of hair and fingernails or for other unproven health reasons.
In a small 2017 study in the Journal of the American Medical Association, healthy adults took 10 milligrams of biotin a day (a commonly used megadose, about 300 times the daily RDA) for a week and then underwent a variety of common immunoassay blood tests, including some for thyroid hormones. Some results came in falsely high, others falsely low, which could result in misdiagnosis and incorrect treatment.
The problem is that biotin is a critical component of some immunoassay tests, and thus high blood levels of the vitamin can lead to inaccurate results. If you take biotin and are having blood tests, ask your doctor to check if any of them are affected by biotin. If so, you may be advised to stop taking the supplement for at least a week before being tested—or stop it altogether, since it’s unlikely to be doing you any good.