Parathyroid hormone (PTH) increases serum calcium levels by stimulating osteoclast-mediated bone resorption, enhancing renal calcium reabsorption, and promoting activation of vitamin D in the kidneys, which increases intestinal calcium absorption. This coordinated action raises blood calcium levels to maintain homeostasis. In contrast, calcitonin lowers serum calcium, and insulin and aldosterone have no direct role in calcium regulation.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.