Calcitonin is a 32 amino acid peptide hormone synthesized by the parafollicular cells of the thyroid. It causes a rapid, but short lived, reduction in serum calcium and phosphate by promoting the incorporation of those ions in the bones. This effect is opposite to that of parathyroid hormone. Staining for calcitonin may be used for the identification of a spectrum of C cell proliferative abnormalities ranging from C cell hyperplasia to invasive tumors. Staining for calcitonin in medullary carcinoma of the thyroid produces a fine granular pattern in the cytoplasm. Amyloid deposits within the tumor may also exhibit varying degrees of calcitonin activity.