What hormone is responsible for the retention of calcium and phosphorus in bone?

The hormone that is primarily responsible for the retention of calcium and phosphorus in bone is calcitonin. This peptide hormone is secreted by the parafollicular cells (also known as C cells) of the thyroid gland and plays a vital role in regulating calcium levels in the blood. When blood calcium levels are elevated, calcitonin is released, which inhibits the activity of osteoclasts—cells that break down bone. By reducing osteoclast activity, calcitonin encourages the uptake of calcium and phosphorus into the bone matrix, thus helping to lower blood calcium levels.

In contrast, parathyroid hormone (PTH) serves a different function. PTH increases blood calcium levels by stimulating osteoclast activity and promoting calcium release from the bones, as well as enhancing calcium reabsorption in the kidneys. Thyroid hormone, while important for general metabolism, does not directly regulate calcium and phosphorus retention in bones. Aldosterone is primarily involved in sodium and water balance and does not play a significant role in bone metabolism or mineral retention. Therefore, calcitonin is the correct answer as it directly promotes the retention of calcium and phosphorus in bone structure.