The aim of this review article is to provide an overview of available literature data on serum basal calcitonin (bCT) and stimulated calcitonin (sCT) cut-off values, highlighting them as a valuable diagnostic tool in everyday thyroidology practice. During the diagnostic evaluation of thyroid nodular disease, mildly elevated bCT values ​​are frequently encountered. If bCT values ​​remain above the reference range but are <100 pg/ml on repeated measurements, further evaluation is warranted. In addition to the usual diagnostic procedure for thyroid nodular disease (medical history, family history of thyroid malignancy, clinical examination, thyroid function tests, and thyroid ultrasonography), a stimulation test, most often a calcium test, is performed to assess the response by measuring the sCT levels. A precise diagnostic threshold, the so-called cut-off for clinically significant pathological values ​​of bCT and sCT that indicate medullary thyroid carcinoma (MTC), is still lacking. However, there are studies that recommend appropriate cut-off values ​​for bCT and sCT that allow a clear and safe distinction between healthy individuals and individuals with C-cell hyperplasia (CCH) from those with MTC. The aim of this review article is to present the latest literature data on existing cut-off values ​​for bCT and sCT in blood, which can certainly be useful to clinicians in everyday practice.