RESUMEN
Introduction: Biotin treatment causes false-low or false-high results in some immunoassays methods. This phenomenon is called as biotin interference. In the present article, a seven-month-old male, with renal failure and laboratory hyperthyroidism due to biotin interference is presented. Case report: High free T4 (fT4), free T3 (fT3), anti-thyroid peroxidase antibody (anti-TPO), anti-thyroglobulin antibody (anti-TG) and low thyroid stimulating hormone (TSH) levels were detected in a seven-month-old male patient who has metabolic acidosis, renal failure, and suspected of metabolic disease. Anti-thyroid drug therapy was started. However, when he was re-evaluated due to the absence of euthyroidism with anti-thyroid therapy (methimazole 0.8 mg/kg /day), it was found that the patient had been given 20 mg/day biotin for acidosis for two months. Biotin interference was considered in hormone measurement. Thyroid function tests were found to be normal 12 days after discontinuation of biotin therapy. Conclusion: Immunoassay measurements which use biotin should be done 2-7days after the last dose of biotin in patients under biotin treatment, but this time may need be much longer in renal failure patients. During this period or if the biotin therapy cannot be stopped, alternative methods should be preferred for analysis.