What is the primary regulator of extracellular calcium?

• A. Calcitonin
• B. Vitamin D
• C. Calcium
• D. PTH

Answer: (D)

Parathyroid hormone is the main regulator of extracellular calcium. When serum calcium drops, the parathyroid glands release PTH, which raises calcium levels through three actions: it stimulates bone resorption to release calcium, increases renal reabsorption of calcium, and promotes the production of active vitamin D (which boosts intestinal calcium absorption). Calcitonin lowers calcium but plays only a minor role in humans, so it isn’t the primary regulator. Vitamin D helps calcium absorption, but its effects depend on PTH-driven activation and regulation; without PTH, calcium homeostasis cannot be maintained effectively. The system also uses a feedback loop: low calcium stimulates PTH release, while high calcium suppresses it.