The Pitfalls of Calcitonin as a Tumor Marker: Real-Life Data of Patients with Elevated Basal Calcitonin Levels but Without Evidence of Medullary Thyroid Carcinoma

Background: Calcitonin, a tumor marker primarily used to diagnose medullary thyroid carcinoma (MTC), can also be elevated in other conditions, complicating diagnosis. This study aims to provide a clinical evaluation of the real-world consequences of unexplained calcitonin elevation. Methods: We conducted a retrospective cohort study of patients with elevated basal calcitonin levels who presented at the Department of General, Visceral, and Transplantation Surgery, University Medical Center Mainz, between January 2015 and March 2025. Additionally, we reviewed electronic health records from 2007 onward for patients with ICD codes indicating calcitonin hypersecretion. Patients with confirmed MTC or genetic syndromes were excluded. Results: Of 345 patients with elevated calcitonin levels, 167 (48%) met the inclusion criteria, and 29 additional patients with calcitonin hypersecretion were identified via ICD, resulting in 167 patients analyzed. More than half of the patients were female (52%), had an average age of 53.9 years and a high prevalence of goiter (86%). Calcitonin levels were slightly elevated (