Which hormone primarily stimulates osteoclast activity to increase blood calcium?

• A. Calcitonin
• B. Vitamin D
• C. Parathyroid hormone
• D. Calcitonin gene-related peptide

Answer: (C)

Calcium homeostasis is controlled by hormones that regulate bone resorption. The parathyroid hormone is the main driver of osteoclast-mediated bone breakdown, which raises blood calcium. It acts on osteoblasts to increase the expression of RANKL (and decrease inhibitors like OPG), promoting differentiation and activation of osteoclasts. This boosts the release of calcium from bone into the bloodstream. While PTH also enhances kidney calcium reabsorption and, via calcitriol, intestinal calcium absorption, its direct effect on osteoclasts is through the RANKL pathway, making it the primary stimulant of osteoclast activity. Calcitonin inhibits osteoclasts, reducing calcium release; vitamin D mainly increases gut calcium absorption and supports mineralization; the other peptide is not the main regulator of bone resorption.