What function does calcitonin serve in calcium homeostasis?

• A. It increases blood calcium levels
• B. It inhibits bone resorption
• C. It stimulates bone formation
• D. It has no significant effect on calcium

Answer: (B)

Calcitonin plays a crucial role in calcium homeostasis primarily by inhibiting bone resorption. Produced by the parafollicular cells (C cells) of the thyroid gland, calcitonin acts to lower blood calcium levels when they become elevated. It accomplishes this by targeting osteoclasts, the cells responsible for breaking down bone tissue, effectively reducing their activity. This action slows down the release of calcium from bones into the bloodstream.

Moreover, calcitonin also promotes the uptake of calcium into the bones, although its direct role in stimulating bone formation is not as pronounced as that of other hormones like parathyroid hormone (PTH) or calcitriol, which are more active in promoting bone formation processes. Consequently, the primary and well-documented effect of calcitonin relating to calcium homeostasis is its ability to inhibit bone resorption, thereby maintaining lower blood calcium levels and contributing to overall calcium regulation in the body.