Calcitonin

Calcitonin is a peptide hormone produced primarily by the parafollicular C cells of the thyroid gland in humans. The human CALCA gene encodes calcitonin, and alternative splicing yields related peptides such as calcitonin gene-related peptide (CGRP). Calcitonin acts to lower elevated blood calcium levels and, to a lesser extent, phosphate.

Its main physiological role is to inhibit osteoclast-mediated bone resorption, reducing the release of calcium from bone. It also promotes the excretion of calcium and phosphate by the kidneys, though its overall influence on calcium homeostasis in humans is modest compared with parathyroid hormone (PTH) and vitamin D. Calcitonin binds to the calcitonin receptor, a G protein–coupled receptor expressed on osteoclasts and renal tubules, triggering signaling pathways that suppress bone resorption and affect renal calcium handling.

Clinically, salmon calcitonin has been used as a drug to treat hypercalcemia, osteoporosis, and Paget disease of bone, available in injectable or nasal spray forms. Its use for osteoporosis has declined due to limited evidence of fracture risk reduction and safety concerns, including reports of increased cancer risk with long-term nasal calcitonin use; many guidelines now favor other agents such as bisphosphonates.

Calcitonin can also be measured in blood as a tumor marker; markedly elevated levels are associated with medullary thyroid carcinoma and, less commonly, C-cell hyperplasia.