What hormone helps to reduce blood calcium levels by promoting its incorporation into bone?

The hormone responsible for reducing blood calcium levels by promoting its incorporation into bone is calcitonin. Calcitonin is produced by the parafollicular cells (C cells) of the thyroid gland and plays a critical role in calcium homeostasis. When blood calcium levels are elevated, calcitonin is released to help lower those levels, primarily by inhibiting the activity of osteoclasts, the cells that break down bone and release calcium into the bloodstream. By doing so, calcitonin encourages the uptake of calcium into the bone matrix, effectively reducing the concentration of calcium in the bloodstream.

In contrast, parathyroid hormone (PTH) increases blood calcium levels by stimulating osteoclast activity and promoting calcium reabsorption in the kidneys. Vitamin D, through its active form calcitriol, also works to increase blood calcium by enhancing intestinal absorption of calcium and promoting its mobilization from the bone. Thyroxine (T4) primarily regulates metabolism and has minimal direct effects on calcium levels compared to calcitonin and PTH. Therefore, calcitonin is the correct hormone that specifically aids in lowering blood calcium levels through bone incorporation.