Calcitonin inhibits which cells and what is the net effect on calcium?

• A. Inhibits osteoclast-mediated bone resorption and lowers serum calcium but physiologic significance is limited.
• B. Inhibits osteoblasts to reduce bone formation.
• C. Stimulates calcium reabsorption in kidney.
• D. Increases intestinal calcium absorption.

Answer: (A)

Calcitonin lowers serum calcium mainly by inhibiting osteoclast-mediated bone resorption. By acting on osteoclasts, it reduces bone breakdown and the release of calcium from bone into the bloodstream, leading to a fall in blood calcium levels. It does not primarily inhibit osteoblasts, nor does it significantly increase renal calcium reabsorption or intestinal calcium absorption—the latter two are mainly controlled by PTH and vitamin D. In humans, the physiologic significance of calcitonin is relatively small, so its ability to lower calcium is modest and most evident in hypercalcemic states rather than normal physiology.