In this case scenario, we outline a situation where biotin causes thyroid problems. It highlights the importance of recognizing the use of OTC and supplement medications.
A 46-year-old woman with a history of primary hypothyroidism presents to her primary care clinic for routine follow-up. She has been clinically stable for several years on levothyroxine (Synthroid) 100 mcg daily, with previously normal TSH values and no recent dose adjustments. At this visit, she reports mild fatigue but denies palpitations, weight changes, heat intolerance, tremor, or anxiety.
Routine thyroid function tests are obtained. The results show a TSH of 0.08 mIU/L with a free T4 of 1.9 ng/dL, placing her free T4 in the high-normal range. Based on these findings, the patient appears to be over-replaced with levothyroxine despite minimal symptoms suggestive of hyperthyroidism.
Concerned about possible drug-induced hyperthyroidism, the clinician reduces the patient’s levothyroxine dose to 75 mcg daily. Over the following four to six weeks, the patient develops worsening fatigue, cold intolerance, constipation, and difficulty concentrating. These symptoms prompt repeat laboratory testing, which now reveals a TSH of 6.8 mIU/L and a free T4 of 0.7 ng/dL, consistent with undertreated hypothyroidism.
A more detailed medication and supplement history is obtained at this time. The pharmacist discovers that the patient recently started an over-the-counter hair, skin, and nails supplement containing biotin 10,000 mcg daily. She began taking the supplement approximately one month before the initial thyroid labs were drawn and did not stop it prior to testing.
Biotin is known to interfere with biotin–streptavidin immunoassays commonly used for thyroid testing. This interference can cause falsely low TSH levels and falsely elevated free T4 levels, creating the appearance of hyperthyroidism when thyroid function is actually stable. In this case, the misleading lab results led to an inappropriate reduction in levothyroxine dosing.
The biotin supplement is discontinued, and the patient is instructed to hold biotin for at least 48 to 72 hours before repeat thyroid testing. Her levothyroxine dose is returned to the previous stable dose of 100 mcg daily. Repeat labs obtained after the biotin washout period show a TSH of 2.1 mIU/L and a free T4 of 1.2 ng/dL. Her symptoms gradually resolve over the next several weeks.
This case highlights how high-dose biotin supplementation can invalidate thyroid laboratory results (excellent board exam question) and lead to inappropriate medication changes. When laboratory findings do not match the clinical picture, clinicians should specifically ask about over-the-counter supplements and consider drug interference before adjusting levothyroxine therapy.
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