Calcitonin serodiagnostics are generally employed for clarification of scintigraphically cold thyroid nodules. The calcitonin level in serum is strongly increased in medullary thyroid carcinoma (C-cell carcinoma). Hyperparathyroidism is present in 20% to 30% of patients with a C-cell carcinoma. Furthermore, calcitonin serodiagnostics are required in paraneoplastic or reactive hypercalcaemia and for monitoring and relapse monitoring following surgical removal of C-cell carcinoma. Unspecifically increased values can be measured in the final trimester of pregnancy, in people taking contraceptives and after calcium infusion. Low calcitonin levels are associated with osteopenia, a preliminary stage of osteoporosis, which is why calcitonin is among the substances given in osteoporosis therapy.