What is the role of calcitonin in human calcium homeostasis?

• A. It inhibits osteoclast-mediated bone resorption and lowers serum calcium but physiologic significance is limited.
• B. It stimulates osteoclasts to release calcium.
• C. It increases intestinal calcium absorption.
• D. It directly raises PTH levels.

Answer: (A)

Calcitonin reduces serum calcium mainly by blocking osteoclast activity in bone, which slows calcium release from bone into the bloodstream. In humans, this effect exists but is relatively small in the overall regulation of calcium; PTH and active vitamin D are the dominant regulators. So the statement that calcitonin inhibits osteoclast-mediated bone resorption and lowers serum calcium, with limited physiologic significance, best captures its role. It does not stimulate osteoclasts, it does not increase intestinal calcium absorption, and it does not directly raise PTH levels.