The Parathyroid hormone (PTH) is the primary regulator of calcium homeostasis in the body. It increases blood calcium levels by stimulating osteoclast-mediated bone resorption, enhancing renal calcium reabsorption, and promoting the activation of vitamin D in the kidneys, which in turn increases intestinal calcium absorption. This tight regulation ensures stable serum calcium, crucial for neuromuscular function and bone metabolism. None of the other options directly regulate calcium levels.

Reference: Guyton and Hall Textbook of Medical Physiology, John E. Hall, 14th Edition.