ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Endocrine Abstracts (2019) 63 P753 | DOI: 10.1530/endoabs.63.P753

Falsely high serum calcitonin levels

Mirjana Stojkovic, Biljana Beleslin, Jasmina Ciric, Milos Stojanovic & Milos Zarkovic

Clinic of Endocrinology, Diabetes and Metabolic Diseases, Clinical Center od Serbia, Belgrade, Serbia.

Calcitonin is a polypeptide hormone synthesized and secreted by the parafollicular cells of the thyroid gland, and has been considered as a good marker for medullary thyroid carcinoma. But several physiologic and pathologic conditions other than medullary thyroid carcinoma have been associated with increased levels of calcitonin. A 39-year old woman was admitted to our hospital for further testing due to high calcitonin level (467 ng/L) measured by immunochemiluminometric assay (ICMA). She had a history of multinodular goiter in past eight years, and calcitonin level was measured for the first time one month before the admission. She had no history of any other disease, and she did not take any medication. On admission, her calcium and phosphate levels, as well as PTH were with in normal range. She had mild hypovitaminosis D (Ca 2.21 mmol/L; PO4 1.15 mmol/L; PTH 30.15 pg/mL; 25OHD3 44.4 nmol/L). Her calcitonin levels were still high measured repeatedly by ICMA (244 ng/L; 220; 229 ng/L), while in normal range (5.3 ng/L) measured by immunoradiometric assay (IRMA). CEA (0.7 ng/mL) and chromogranin A (29.9 ng/mL) were in normal reference range. An ultrasound examination showed multinodular goiter with four heteroechoic, solid nodules in both lobes, size 19 mm, 15 mm, 12 mm and 11 mm, with no suspicious characteristics. An ultrasound guided fine needle biopsy was performed and cytopathologic result was benign (colloid nodules). Dilution of patient’s serum 1:3, 1:5, 1:10 showed lost of linearity of calcitonin values measured by ICMA (1240; 85; 61 ng/L). Calcium stimulation test (2 mg/kg of elemental Ca, iv inf. during 5 minutes) showed persistently elevated levels of calcitonin, with absence of calcitonin response and domed calcitonin curve measured by ICMA (258.0; 268.0; 263.0; 252.0; 241.0 ng/L), while normal stimulation curve of calcitonin measured by IRMA (5,9; 9,0; 15,7; 9,2; 6,6 ng/L). We concluded that the calcitonin levels measured by ICMA were falsely elevated, probably due to presence of heterophilic antibodies, and that calcitonin measured by IRMA was her true calcitonin level. We followed patient annually for the next four years with ultrasound scan and measuring calcitonin by IRMA. Her calcitonin levels were with in normal range (4.4; 4.2; 2.5 ng/L). Due to nodular growth, patient was sent on thyroidectomy in december 2018. Patohistological diagnosis was: struma colloides cystica partim hyperplastica.

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