How do the parathyroid glands respond when blood calcium levels fall?

When blood calcium levels fall, the parathyroid glands respond through a sensitive feedback mechanism to restore homeostasis. The process functions as follows:

• Detection: Calcium-sensing receptors on the surface of the parathyroid cells monitor the concentration of ionized calcium in the blood. When these levels decrease, reduced receptor binding triggers an almost immediate increase in the synthesis and secretion of parathyroid hormone (PTH).123
• Bone Mobilization: PTH stimulates osteoblasts to recruit and activate osteoclasts, which break down bone tissue to release calcium and phosphorus into the circulation.12
• Renal Regulation: PTH acts on the kidneys to increase the reabsorption of calcium from urine back into the blood. Simultaneously, it promotes the excretion of phosphate, which helps prevent the formation of insoluble calcium-phosphate complexes.123
• Vitamin D Activation: PTH stimulates the kidneys to convert calcidiol into calcitriol, the active form of vitamin D. This hormone then increases the efficiency of calcium absorption in the gastrointestinal tract.123