Calcitonin as Biomarker for the Medullary Thyroid Carcinoma.

Calcitonin (CTN) is a polypeptide hormone consisting of 32 amino acids with a disulfide bridge between position 1 and 7 that is mainly produced by the C-cells of thyroid gland. The measurement of CTN concentrations in blood reflects C-cell activity and is performed in general by immunoassay methods. However, there are analytical, physiological, pharmacological, and pathological factors that can influence results of serum CTN values. Due to the influence of these factors there is a high variability in assay-dependent cutoffs used to discriminate between MTC, C-cell hyperplasia (CCH) and the absence of the pathological impairment of C cells. There is a lot of evidence that the measurement of serum CTN concentrations in patients with thyroid nodules can lead to an earlier diagnosis of MTC or CCH than the exclusive use of imaging procedures and/or fine needle aspiration cytology. Basal CTN concentrations higher than 60-100 pg/mL are highly indicative for the diagnosis MTC. In the range between cutoff and 60 pg/mL CTN, both MTC and HCC may be a relevant diagnosis. Procalcitonin (PCT) and CTN appear to have a comparable diagnostic capability to diagnose MTCs. However, "positive" PCT values more than 50 pg/mL may be reached also in subclinical infections and will lead, therefore, to an overdiagnosis of the tumor. Calcium-stimulated serum CTN concentrations higher than cutoff values could improve diagnostics of MTC but a lack of replicable cutoff values in different studies favors the use of only basal values, currently.