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How does parathyroid hormone regulate calcium levels in the body?

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bone disease Defect of tibia, caused by septic osteomyelitis in childhood, with compensatory thickening of the fibula (right). Normal bones are shown at left.

Parathyroid hormone (PTH) is essential for maintaining calcium homeostasis in the body. Its secretion is regulated by a negative feedback loop: when serum calcium levels drop, calcium-sensing receptors on the parathyroid glands trigger an increase in PTH release.12

PTH elevates serum calcium concentrations through three primary mechanisms:

Bone Resorption: PTH stimulates osteoclasts—large cells that break down bone tissue—to mobilize and release calcium into the bloodstream.13
Kidney Reabsorption: It acts on the kidneys to enhance the reabsorption of calcium from urine, preventing its loss from the body.13
Vitamin D Activation: PTH stimulates the kidneys to convert vitamin D into its most active form, calcitriol. Calcitriol then increases the efficiency of calcium absorption from the gastrointestinal tract.13

Additionally, PTH promotes the excretion of phosphate by the kidneys. By lowering serum phosphate levels, PTH reduces the formation of insoluble calcium-phosphate complexes, further supporting the availability of free, ionized calcium in the blood.14