Biotin and Thyroid Function Test: A Case Study

A 55-year-old woman was referred to an endocrinology clinic with low thyroid-stimulating hormone (TSH) levels but normal free T4, experiencing hair thinning and an abnormal smell of 'smoke', without other symptoms of thyroid dysfunction. She had a history of COVID-19 and had been taking 300 micrograms of biotin daily for six months to address hair loss. Physical examination showed no signs of thyroid abnormalities, and her thyroid function tests (TFTs) were normal before starting biotin. Subsequent tests confirmed low TSH but normal free T4 and T3, and thyroid receptor antibodies were negative. After discontinuing biotin for three days, her TFTs returned to normal. The case illustrates that high-dose biotin can interfere with TFTs in vitro, leading to misdiagnosis of thyroid conditions. The American Thyroid Association recommends discontinuing biotin two days before TFTs to avoid misinterpretation of results, unnecessary investigations, patient anxiety, and healthcare costs.