Calcitonin, eel is the thyroid hormone peptide that contributes to the regulation of calcium homeostasis, widely used in the research of postmenopausal osteoporosis. Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid gland in humans, and in many other animals in the ultimopharyngeal body. Calcitonin acts to reduce blood calcium ions, opposing the effects of parathyroid hormone. Calcitonin assay is used in identifying patients with nodular thyroid diseases. It is helpful in making an early diagnosis of medullary carcinoma of thyroid. A malignancy of the parafollicular cells, i.e. Medullary thyroid cancer, typically produces an elevated serum calcitonin level. Prognosis of medullary thyroid cancer depends on early detection and treatment.