Calcitonin is secreted by the C-cells, or parafollicular cells, of the thyroid gland. The thyroid is a butterfly-shaped gland located in the front of the neck. Calcitonin is a hormone that helps regulate calcium levels in the blood. It's released in response to high levels of calcium in the blood, and it works by: Inhibiting the activity of osteoclasts, which are cells that break down bone and release calcium into the bloodstream Increasing the excretion of calcium by the kidneys Calcitonin also opposes the actions of parathyroid hormone, which increases blood calcium levels.