calcitonin

Calcitonin is the thyroid’s side job — a hormone about calcium rather than metabolism. Its name literally means it tones down calcium, nudging the level in the blood downward, the mirror image of parathyroid hormone’s job.

It is made not by the follicular cells but by separate parafollicular C cells scattered in the thyroid. When blood calcium rises, calcitonin is released and mainly acts on bone, restraining the osteoclasts that dissolve bone and release calcium. By slowing this bone breakdown, it modestly lowers blood calcium and tempers bone turnover.

In humans, calcitonin is surprisingly minor: people with no thyroid still keep normal calcium, so it is not essential for calcium balance the way parathyroid hormone is. Its main clinical importance is as a tumor marker — markedly high calcitonin points to medullary thyroid cancer, which arises from the C cells. Synthetic calcitonin has also been used as a drug to curb bone loss and pain in certain bone conditions.