Calcitonin is a polypeptide hormone formed by the proteolytic cleavage of a larger prepropeptide. It is produced primarily by the parafollicular C-cells of the thyroid. Calcitonin is a potent plasma calcium-lowering peptide; it decreases the level of calcium and phosphate in blood by promoting the incorporation of these ions in bones. Calcitonin expression is found in C-cell hyperplasia and medullary thyroid carcinomas. It is a useful marker in identification of C-cell proliferative abnormalities, and for distinguishing medullary carcinoma from papillary and follicular thyroid cancer.

Other tissues such as the lungs and intestinal tract can also synthesize calcitonin. Expression of Calcitonin in both non-neoplastic and neoplastic neuroendocrine cells of these organs, such as carcinoids and small cell lung carcinomas, may be seen.