Calcitonin is a peptide hormone synthesized by the parafollicular cells, also known as C Cells, of the thyroid gland.
Calcitonin secretion is regulated by and varies proportionally with blood calcium concentration.
When calcium is high, calcitonin secretion increases.
When calcium is low, calcitonin secretion decreases.
Calcitonin appears to be of minor physiological importance in humans as its endocrine effects on calcium concentration is largely overwhelmed by those of parathyroid hormone. Consequently, total removal of the thyroid gland does not result in any significant calcium dysregulation. In general though, calcitonin appears to encourage deposition of bone salts and reduce osteoclast differentiation. Thus, the net effect of calcitonin is to reduce calcium levels.