title: Normal perioperative serum calcitonin levels in patients with advanced medullary thyroid carcinoma: case report and review of the literature.
abstract: BACKGROUND
  Medullary thyroid carcinoma (MTC), a tumor of the parafollicular C cells of the gland, comprises 3-5% of all malignant thyroid neoplasms. Calcitonin, a polypeptidic hormone secreted by the neoplastic cells, is considered a very sensitive and specific MTC tumor marker. Patients with MTC usually present elevated serum calcitonin levels, which correlate with tumor burden and prognosis.
  
  
  OBJECTIVE
  To describe a case of advanced MTC with normal serum calcitonin and review the literature on this subject.
  
  
  METHODS
  A case study was performed.
  
  
  METHODS
  There were no interventions.
  
  
  METHODS
  A case of advanced MTC with normal serum calcitonin was studied.
  
  
  RESULTS
  Serum calcitonin was measured by two distinct assays, a chemiluminescent immunometric and an in-house two-site monoclonal antibody-based immunofluorometric assay. To rule out a "hook effect," or posttranslational modifications of calcitonin molecule, serum dilutions and tumor immunohistochemistry for calcitonin with the same antibodies used for serum calcitonin measurements were performed. Serum calcitonin levels were within the normal range in both assays, whereas the tumor stained strongly positive for calcitonin. These findings suggest that the tumor was able to produce but not to secrete the calcitonin protein. Five other cases of advanced MTC with normal serum calcitonin levels had been previously reported.
  
  
  CONCLUSIONS
  We present an unusual case of advanced MTC with normal serum calcitonin levels. Awareness of MTC cases presenting with normal serum calcitonin levels is important in clinical practice and is particularly relevant to centers that use this test for screening.
authors.affiliation: Thyroid Section, Endocrine Division, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil.
authors.forename: José Miguel
authors.initials: JM
authors.lastname: Dora
authors.forename: Maria Heloisa Busi da Silva
authors.initials: MH
authors.lastname: Canalli
authors.forename: Clarissa
authors.initials: C
authors.lastname: Capp
authors.forename: Márcia Khaled
authors.initials: MK
authors.lastname: Puñales
authors.forename: José Gilberto H
authors.initials: JG
authors.lastname: Vieira
authors.forename: Ana Luiza
authors.initials: AL
authors.lastname: Maia
mesh_terms: D000328:Adult; D014408:Biomarkers, Tumor / Q000097:blood*; D002116:Calcitonin / Q000097:blood*; D018276:Carcinoma, Medullary / Q000097:blood* / Q000601:surgery; D006801:Humans; D008297:Male; D013964:Thyroid Neoplasms / Q000097:blood* / Q000601:surgery
chemical_list: D014408:Biomarkers, Tumor; D002116:Calcitonin