A case of spurious hypercalcitoninemia: A cautionary tale on the use of plasma calcitonin assays in the screening of patients with thyroid nodules for neoplasia

The measurement of plasma CT has an important role as a screening test for medullary thyroid carcinoma (MTC) in patients with thyroid nodules. However , elevated plasma CT levels should be interpreted within the context of the overall clinical picture in each individual case and carefully validated b efore therapeutic decisions are made. We present the case of a 17-yr-old gi rl who was referred to us with a thyroid nodule and elevated plasma CT leve ls, as measured by a one-site RIA not involving prior plasma extraction. Pl asma CT was re-measured using two different methods, a RIA with prior plasm a extraction and a two-site immunochemiluminometric assay (ICMA), and was e ither very low or undetectable. Subsequently, samples were re-assayed using the initially applied CT RIA; plasma CT levels were again found to be elev ated. These elevations were of a spurious nature, probably caused by the pr esence of an unidentified substance in the patient's plasma interfering wit h the measurement of CT in the initially used RIA. Our patient was eventual ly diagnosed with Hashimoto's thyroiditis, and had no evidence of MTC. As s everal conditions can cause either true or spurious hypercalcitoninemia, we suggest that elevated plasma CT levels should be confirmed at least once b efore other extensive diagnostic investigations are initiated or thyroidect omy is recommended. Finally, the assay selected should detect only the matu re CT molecule.