Calcitonin is a 32-amino-acid polypeptide that is secreted primarily by the parafollicular C-cells. Its main biological effect is to inhibit osteoclastic bone resorption. This property has led to Calcitonin’s use for disorders characterized by increased resorption such as Paget’s disease and for some patients with osteoporosis. The amino acid sequences of porcine, bovine, salmon, and human Calcitonin are known, and biologically active molecules have been synthesized. The structure of each is similar, whereas the immunologic activity of Calcitonin from various species differs profoundly. The most prominent clinical syndrome associated with a disordered hypersecretion of Calcitonin is medullary carcinoma of the thyroid (MTC). MTC is a tumor of the Calcitonin producing C-cells of the thyroid gland. MTC comprises 5 - 10% of all thyroid cancer. It may occur sporadically or in a familial form that is transmitted as an autosomal dominant trait. Despite its being a relatively uncommon tumor, MTC has great clinical importance because of its familial distribution. Further, it lent itself to be diagnosed early by serum Calcitonin. This is frequently associated with other clinical features and it has good potential for cure with surgery. Although a rare tumor, it can occur in a familial pattern as a Type II multiple endocrine neoplasia. These tumors usually produce diagnostically elevated serum concentrations of Calcitonin