Spurious hypercalcitoninemia in patients with nodular thyroid disease induced by heterophilic antibodies

Background. Serum calcitonin is the most useful tumor marker for the diagnosis and follow-up of medullary thyroid carcinoma (MTC). Spurious hypercalcitoninemia caused by heterophilic antibody interference (HAI) is rarely found in patients without MTC. Methods. We studied 2 patients with hypercalcitoninemia and thyroid nodules, but no evidence of MTC on fine-needle aspiration cytology. We performed calcium stimulation tests, measured serum calcitonin with another calcitonin kit, performed dilution tests, and remeasured serum calcitonin after applying heterophilic blocking tubes. Results. In a 31-year-old woman with no response to the calcium stimulation test, serum calcitonin was <5 pg/mL using another kit. After we applied heterophilic blocking tubes, the serum calcitonin level decreased to normal range. We concluded that patient had spurious hypercalcitoninemia. In a 63-year-old woman, all tests revealed that the patient had true hypercalcitoninemia. The patient underwent total thyroidectomy that revealed MTC. Conclusions. We suggest that patients suspected for spurious hypercalcitoninemia should undergo further investigation due to HAI.