Calcitonin and procalcitonin: Revisiting the overlooked role of C cells.

Calcitonin (CT) is a hormone produced by C cells in the thyroid gland. Its primary function is to regulate bone turnover. However, it is believed to be of little importance to human physiology because its absence following thyroidectomy has no dramatic effects. It was used in the treatment of osteoporosis but has now largely been replaced by bisphosphonates and monoclonal antibodies. However, some studies suggest that CT may have additional functions, such as those related to bone structure, osteoprotection, and pain management. This review summarizes CT synthesis and function and discusses its role and that of its precursor, procalcitonin, as biomarkers. Procalcitonin detection has advantages over some established markers in sepsis management and due to its greater stability, it is also an alternative to CT for managing medullary thyroid carcinoma. Recent research has raised the possibility that procalcitonin could serve as a direct molecular target for treating sepsis. Potential roles of various regulatory peptides released by C cells that may contribute to paracrine fine-tuning of thyroid hormone secretion by follicular thyrocytes are considered. Health-care providers should inform patients that despite optimal thyroxine replacement therapy, subtle symptoms may still occur due to the absence of C cells.