Which hormone is primarily responsible for lowering blood calcium levels?

The hormone primarily responsible for lowering blood calcium levels is calcitonin. Calcitonin is produced by the parafollicular cells (also known as C cells) of the thyroid gland. Its main function is to inhibit the activity of osteoclasts, which are cells that break down bone and release calcium into the bloodstream. By inhibiting these cells, calcitonin decreases the release of calcium from the bones, thereby reducing blood calcium levels.

Additionally, calcitonin increases the excretion of calcium in the kidneys, contributing further to the lowering of serum calcium levels. This process is particularly important during periods when calcium levels in the blood are elevated, helping to maintain calcium homeostasis in the body.

In contrast, parathyroid hormone, which is produced by the parathyroid glands, has the opposite effect by increasing blood calcium levels. Insulin and glucagon are hormones that primarily influence glucose metabolism and have no direct role in the regulation of blood calcium levels. Therefore, calcitonin is the correct choice for the hormone that lowers blood calcium levels.