SUMMARY Calcitonin (CT) is a diagnostic and follow-up marker of medullary thyroid carcinoma. Heterophile antibodies (HAbs) may interfere during immunometric assay measurements and result in falsely high CT levels and different markers. A 50-year-old female patient was referred to our institution for elevated CT levels (3,199 pg/mL [0-11,5]). Physical examination and thyroid ultrasonography show no thyroid nodules. Because of the discrepancy between the clinical picture and the laboratory results, various markers and hormones were examined to determine whether there was any interference in the immunometric assay. Thyroglobulin (Tg) and Adrenocorticotropic hormone (ACTH) levels were also found inaccurately elevated. After precipitation with polyethylene glycol, CT, Tg, and ACTH levels markedly decreased, showing macro-aggregates. Also, serial dilutions showed non-linearity in plasma concentrations. Additionally, CT samples were pretreated with a heterophilic blocking tube before measuring, and the CT level decreased to < 0.1 pg/mL, suggesting a HAb presence. Immunoassay interference should be considered when conflicting laboratory data are observed. This may help reduce the amount of unnecessary laboratory and imaging studies and prevent patients from complex diagnostic procedures. (CT followup follow up carcinoma HAbs (HAbs 50yearold yearold 50 year old 3,199 3199 3 199 (3,19 pgmL pg mL 011,5. 0115 0 11,5 . 11 5 [0-11,5]) nodules results Tg (Tg (ACTH glycol macroaggregates. macroaggregates macro aggregates. aggregates macro-aggregates Also nonlinearity non linearity concentrations Additionally measuring 01 1 0. presence observed procedures 3,19 319 19 (3,1 011 011,5 115 11, [0-11,5] 3,1 31 (3, 011, [0-11,5 3, (3 [0-11, ( [0-11 [0-1 [0- [0 [