Which hormone is responsible for transferring calcium from bone to blood?

The hormone responsible for transferring calcium from bone to blood is parathyroid hormone (PTH). PTH is produced by the parathyroid glands and plays a crucial role in calcium homeostasis. When blood calcium levels are low, PTH is released into the bloodstream, which stimulates osteoclasts—cells that break down bone tissue. This process releases stored calcium from the bone into the blood, effectively increasing serum calcium levels.

Additionally, PTH promotes the reabsorption of calcium in the kidneys, reducing urinary excretion, and stimulates the activation of vitamin D, which enhances intestinal absorption of calcium from dietary sources. Together, these actions help maintain appropriate calcium levels in the blood, which are vital for various physiological processes, including muscle function and nerve signaling.

The other hormones listed, such as calcitonin, insulin, and cortisol, have different functions in the body. Calcitonin acts to lower blood calcium levels by inhibiting osteoclast activity, insulin primarily regulates glucose metabolism, and cortisol is involved in stress responses and has a broader regulatory role in metabolism, but does not directly transfer calcium from bone to blood.