Biotin and thyroid function test

ABSTRACT

Introduction: Biotin is a vitamin-B supplement and is sold over-the-counter alone or in combination with other vitamins and minerals to improve skin, nail, and hair health. Exogenous biotin intake is becoming a common cause of apparent abnormal thyroid function tests in endocrine practice. We present a case report. Case Description A 55-year-old lady was referred to endocrinology clinic for low TSH but normal fT4. The only symptom she had was abnormal smell of ‘smoke’. She had no ongoing symptoms of thyrotoxicosis or hypothyroidism apart from hair thinning. She had COVID-19 infection with pyrexia and constitutional symptoms a year ago. There was no other past medical or family history of note. On direct questioning, she admitted taking biotin 300ug every day in preceding six months for hair loss. Examination revealed euthyroidism and no goitre. TFTs prior to commencing biotin were normal. Further TFTs confirmed low TSH but normal fT4 and fT3 and thyroid receptor antibody was negative. TFTs off biotin for three days were normal and she was reassured. Discussion: There is no in vivo relationship between biotin and thyroid hormones, but high dose biotin affects thyroid function test in vitro when assessed with immunoassays using streptavidin-biotin immobilizing system. Apparently low TSH with or without raised T4 suggests thyrotoxicosis or thyroxine over-replacement. All patients presenting with such abnormal thyroid function tests should be questioned about biotin intake along with iodine as many multivitamins contain high dose biotin and patients are often unaware. This can avoid unnecessary investigations, patient anxiety, and healthcare costs. American Thyroid Association advises stopping biotin two days before thyroid function test. Patients can continue to take biotin except temporarily before thyroid blood tests.