Biotin is a B vitamin found in vitamin B complex supplements. It's also sold as an independent supplement and many people take megadoses to boost hair growth or improve energy.  Biotin is inactivated by egg whites. Thus many people on low carbohydrate diets who regularly eat egg whites have also been advised to take megadoses of biotin.

Several laboratory bioassays, including those that measure thyroid hormone, TSH (the pituitary hormone thyroid stimulating hormone or thyrotropin), and thyroid binding inhibitory immunoglobulins (TBII, used to diagnose Graves' disease) employ a biotin-streptavidin affinity in their design. Thus biotin in serum samples causes falsely increased thyroid hormone levels (FT4, FT3), TBII and a falsely low TSH.

In one recent journal article, a patient with progressive multiple sclerosis was found have an extremely high FT4, T3, TBII and a low TSH leading to a diagnosis of severe Graves' disease. Because the patient had not symptoms of hyperthyroidism a review of his medications was made, which revealed a megadose of biotin. The diagnosis was made in error based on erroneous laboratory test results.

The authors of this report recommended tha biotin supplements should be stopped for at least two days prior to blood tests using biotin-sensitive tests to avoid major misdiagnoses.  Because most hormone and tumor marker assays use these types of immunoassays, any unusual blood test results should be considered suspect in patients taking biotin especially at megadoses.  A 100 mg B complex vitamin contains 100 mcg of biotin, whereas most independent biotin supplements contain 3000 to 5000 mcg.

Resource:

Barbesino, G. 2016. "Misdiagnosis of Graves' Disease with Apparent Severe Hyperthyroidism in a Patient Taking Biotin Megadoses. Thyroid, Jun; 26 96): 860-3.