Which of the following hormones is involved in the feedback regulation of blood calcium levels?

Calcitonin is the hormone involved in the regulation of blood calcium levels. It is produced by the parafollicular cells (C cells) of the thyroid gland and plays a crucial role in maintaining calcium homeostasis in the body. When blood calcium levels become elevated, calcitonin is released, leading to a decrease in calcium levels through several mechanisms.

One of its primary actions is to inhibit osteoclast activity in the bones. Osteoclasts are the cells responsible for bone resorption, which releases calcium into the bloodstream. By reducing the activity of these cells, calcitonin helps to lower the amount of calcium released from the bones into the blood, thus decreasing overall blood calcium levels. Additionally, calcitonin promotes the excretion of calcium by the kidneys, further contributing to the lowering of blood calcium levels.

In contrast, the other hormones mentioned do not have a direct role in calcium regulation. Follicle Stimulating Hormone is primarily involved in reproductive processes, melatonin regulates circadian rhythms, and glucagon is primarily involved in glucose metabolism and increasing blood sugar levels. Therefore, calcitonin stands out as the hormone specifically tasked with the feedback regulation of blood calcium levels.