Calcium concentration in the serum is regulated by:

• A. Insulin
• B. Parathyroid hormone
• C. Thyroxine
• D. Vitamin C

Answer: (B)

Calcium concentration in the serum is maintained by a hormonal feedback system centered on parathyroid hormone. When serum calcium falls, the parathyroid glands release PTH. PTH raises blood calcium through three main actions: it stimulates bone resorption to release calcium into the bloodstream; it increases renal reabsorption of calcium, reducing urinary loss; and it enhances production of active vitamin D (calcitriol) by the kidneys, which boosts intestinal absorption of calcium. The combined effect is to restore serum calcium toward its set point. When calcium is high, PTH secretion decreases as part of the feedback. The other substances listed do not regulate calcium levels in this direct, primary way: insulin governs glucose metabolism, thyroxine influences metabolic rate and can affect bone turnover in indirect ways, and vitamin C has no direct role in calcium homeostasis.