Calcitonin

Calcitonin, secreted by thyroid parafollicular C cells, acts as a physiological antagonist to parathyroid hormone by suppressing osteoclastic bone resorption and renal tubular calcium reabsorption. While minor in routine adult calcium homeostasis, serum calcitonin serves as an indispensable diagnostic and post-thyroidectomy surveillance biomarker for medullary thyroid carcinoma, directly correlating with tumor burden, staging, and systemic progression.