Calcitonin is a hormone produced primarily by the parafollicular cells (also known as C-cells) of the thyroid gland. Its primary function is to help regulate calcium levels in the blood, working in opposition to parathyroid hormone (PTH). While not as prominent as other hormones in calcium regulation, it does play a role in lowering blood calcium by inhibiting osteoclast activity in bones and increasing calcium excretion in the kidneys.