Misleading Thyroid Laboratory Results Caused by Biotin Supplementation

Patient Background

A 46-year-old female with a history of primary hypothyroidism presented to her primary care clinic for routine follow-up. She had been clinically stable for several years on levothyroxine 100 mcg daily, with consistently normal thyroid-stimulating hormone (TSH) values and no recent dose adjustments. Her prior thyroid function tests reflected adequate and stable replacement therapy.

Clinical Presentation

At the current visit, the patient reported mild fatigue but denied symptoms suggestive of thyrotoxicosis, including palpitations, weight loss, tremor, heat intolerance, anxiety, or insomnia. Routine thyroid laboratory testing demonstrated a suppressed TSH of 0.08 mIU/L with a free T4 of 1.9 ng/dL, placing the free T4 in the upper-normal range. These findings raised concern for levothyroxine over-replacement, despite minimal supporting clinical symptoms.

Based on the laboratory results, the clinician reduced the levothyroxine dose to 75 mcg daily.

Over the subsequent four to six weeks, the patient developed worsening fatigue, cold intolerance, constipation, and cognitive slowing. Repeat thyroid testing revealed a TSH of 6.8 mIU/L and free T4 of 0.7 ng/dL, consistent with undertreated hypothyroidism.

Clinical Issue Identified

Given the abrupt shift from apparent biochemical hyperthyroidism to overt hypothyroidism following dose reduction, a detailed medication and supplement review was undertaken. The pharmacist identified recent initiation of an over-the-counter hair, skin, and nails supplement containing biotin 10,000 mcg daily, which the patient had begun approximately one month prior to the initial thyroid testing.

High-dose biotin is known to interfere with biotin–streptavidin–based immunoassays commonly used to measure TSH and free thyroid hormones. This interference can result in falsely low TSH values and falsely elevated free T4 levels, mimicking biochemical hyperthyroidism despite stable thyroid function. In this case, assay interference led to misinterpretation of laboratory results and an inappropriate reduction in levothyroxine dosage.

Pharmacist Intervention and Clinical Management

Identification of Laboratory Interference

• Recognize biotin supplementation as the likely cause of discordant thyroid laboratory results.

• Correlate laboratory findings with the absence of clinical hyperthyroid symptoms.

Therapeutic Correction

• Discontinue the biotin-containing supplement.

• Reinstate levothyroxine at the previous stable dose of 100 mcg daily.

Laboratory Reassessment

• Instruct the patient to withhold biotin for at least 48–72 hours prior to repeat thyroid testing.

• Repeat thyroid function tests after adequate biotin washout.

Monitoring and Follow-Up

• Confirm normalization of thyroid parameters (TSH 2.1 mIU/L, free T4 1.2 ng/dL).

• Monitor for gradual resolution of hypothyroid symptoms over subsequent weeks.

• Reinforce the importance of disclosing all over-the-counter supplements prior to laboratory evaluation.

Patient and Provider Education

• Educate the patient regarding supplement-related laboratory interference.

• Communicate findings to the prescriber to prevent future misinterpretation and unnecessary dose changes

Clinical Rationale and Ramifications

This case highlights the critical impact of supplement-induced laboratory interference on clinical decision-making. High-dose biotin supplementation can produce misleading thyroid test results, leading clinicians to incorrectly diagnose over- or under-replacement and adjust levothyroxine dosing inappropriately.

In this patient, reliance on laboratory data without alignment to the clinical picture resulted in iatrogenic hypothyroidism, symptom burden, and avoidable treatment disruption. Failure to recognize assay interference could have led to prolonged symptoms, repeated dose adjustments, and unnecessary diagnostic investigations.

This case underscores the importance of comprehensive medication and supplement reconciliation, particularly when laboratory findings are inconsistent with patient presentation. Clinicians should maintain a high index of suspicion for analytical interference and verify supplement use before altering long-term endocrine therapy. Early recognition preserves therapeutic stability, prevents patient harm, and reinforces rational, evidence-based thyroid management.