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BACKGROUND: Physicians are often guided by laboratory values. When a clinical presentation does not match laboratory values, one must consider the possibility that these values may be falsely increased or decreased. A common cause is analytical interference. CASE DESCRIPTION: A 57-year-old male, presenting with fatigue and palpitations, had high TSH and normal FT4 values. Although there were no fitting clinical symptoms for these values, the patient was treated with levothyroxine assuming he had subclinical hypothyroidism. TSH levels remained high, however, whereas FT4 levels increased and the patient developed thyrotoxicosis. Eventually, it was discovered that the TSH was falsely elevated. CONCLUSION: The patient turned out to have macro TSH, where TSH forms conjunctions with IgG into larger molecules. These conjugates cause a rarely occurring interference during laboratory analysis, resulting in a falsely increased TSH value.