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 before therapeutic decisions are made. We present the case of a 17-yr-old girl who was referred to us with a thyroid nodule and elevated plasma CT levels, as measured by a one-site RIA not involving prior plasma extraction. Plasma CT was re-measured using two different methods, a RIA with prior plasma extraction and a two-site immunochemiluminometric assay (ICMA), and was either very low or undetectable. Subsequently, samples were re-assayed using the initially applied CT RIA; plasma CT levels were again found to be elevated. These elevations were of a spurious nature, probably caused by the presence of an unidentified substance in the patient's plasma interfering with the measurement of CT in the initially used RIA. Our patient was eventually diagnosed with Hashimoto's thyroiditis, and had no evidence of MTC. As several conditions can cause either true or spurious hypercalcitoninemia, we suggest that elevated plasma CT levels should be confirmed at least once before other extensive diagnostic investigations are initiated or thyroidectomy is recommended. Finally, the assay selected should detect only the mature CT molecule.