Analytical errors owing to endogenous interfering substances may occur for some patient specimens. Interference can lead to false-positive or falsenegative results, or both, which may lead to a false diagnosis or inappropriate treatment. We report a case of false elevation of thyroid-stimulating hormone concentrations measured using an electrochemiluminescent immunoassay in a clinically euthyroid 10-year-old Korean boy with central precocious puberty. If the clinical presentations and test results of a patient are inconsistent, a clinician should suspect interference in clinical laboratory tests.