How does parathyroid hormone raise calcium levels in the blood?

Parathyroid hormone (PTH) plays a critical role in maintaining calcium homeostasis by acting through three primary mechanisms to elevate ionized calcium levels in the blood:

• Bone Resorption: PTH stimulates osteoblasts to produce factors that induce the maturation of osteoclasts. These cells break down bone tissue, releasing stored calcium and phosphorus into the circulation.12
• Renal Reabsorption: PTH acts directly on the kidneys to enhance the tubular reabsorption of calcium, preventing it from being lost in the urine.12
• Vitamin D Activation: PTH stimulates the kidneys to convert vitamin D into its most active form, calcitriol. Calcitriol then travels to the small intestine, where it increases the efficiency of dietary calcium absorption into the bloodstream.12

Additionally, PTH lowers serum phosphate levels by inhibiting its renal reabsorption. This is significant because it reduces the formation of insoluble calcium-phosphate complexes, thereby increasing the availability of free, ionized calcium in the blood.2