Calcitonin is a hormone produced and secreted by C-cells (parafollicular cells) in the thyroid gland. Its primary role is to lower blood calcium levels by inhibiting the activity of osteoclasts — the cells that break down bone — and by reducing calcium reabsorption in the kidneys. It acts as a counterbalance to parathyroid hormone (PTH), which raises calcium.

In clinical practice, calcitonin testing is most commonly used as a tumour marker for medullary thyroid carcinoma (MTC), a type of thyroid cancer that arises from C-cells. Significantly elevated calcitonin is a sensitive and specific marker for MTC and is used both for diagnosis and post-treatment monitoring.