Biotin (vitamin B7) is a water-soluble vitamin used as a co-enzyme for carboxylases essential for humanmetabolism. The high affinity to streptavidin makes biotin an important substance in immunoassays. Excessivebiotin intake due to over-the counter supplements has become problematic because of the effects on laboratorytest results. There have been no reports of biotin-induced thyroid immunoassay interference in Korea. We reportthree patients with papillary thyroid cancer who showed false thyrotoxicosis on follow-up laboratory examinationswith a literature review. The patients’ medical history should be thoroughly questioned and patients should beinformed to curtail consuming biotin before laboratory tests to avoid assay interference. Non-biotinylated assayscan be considered if it is impossible to withhold the supplements. These methods will prevent physicians frommaking incorrect decisions that could result in an inappropriate treatment for their patients.