We describe a case of a patient with metastasized differentiated thyroid carcinoma who was treated with total thyroidectomy followed-up by radioactive iodine treatment. During treatment and follow-up the thyroglobulin levels were assayed which surprisingly did not match the clinical condition. An analytical flaw was suspected. Re-analysis in the laboratory showed the presence of a high dose hook effect (HDH), resulting in falsely low Tg levels. This case shows that HDH effects in immunoassays, like the thyroglobulin assay, still exist in daily practice. Discordance between laboratory results and clinical condition underlines the importance of short lines of communication between clinical chemists and medical doctors.