How does calcitonin differ from parathyroid hormone (PTH)?

Calcitonin plays a key role in regulating calcium homeostasis in the body, primarily by reducing the concentration of calcium in the blood. It achieves this by promoting the deposition of calcium into bones and inhibiting the activity of osteoclasts, which are the cells responsible for bone resorption. As a result, calcitonin helps maintain bone density and decreases serum calcium levels.

Parathyroid hormone (PTH), on the other hand, has the opposite function. It is secreted by the parathyroid glands in response to low blood calcium levels and works to increase calcium levels in the blood. PTH stimulates osteoclast activity, leading to the release of calcium from bones into the bloodstream. Additionally, PTH enhances calcium reabsorption in the kidneys and promotes the conversion of vitamin D to its active form, which subsequently increases intestinal absorption of calcium.

The contrast between these two hormones highlights how calcitonin and PTH work antagonistically to keep calcium levels within the narrow range necessary for proper physiological function. The correct answer illustrates this fundamental difference in their roles in calcium regulation—calcitonin's role in adding calcium to bones, while PTH is responsible for increasing calcium in the blood.