Which hormone is primarily responsible for regulating serum calcium levels?

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

Answer: (B)

Calcium homeostasis is governed by hormones, with parathyroid hormone acting as the primary regulator of serum calcium. When calcium levels fall, the parathyroid glands release PTH. It raises calcium by stimulating bone resorption to release calcium into the blood, increasing renal reabsorption of calcium in the kidneys, and promoting the formation of active vitamin D, which enhances intestinal calcium absorption. This coordinated action restores calcium toward normal levels. Other substances listed do not regulate calcium in this primary way: insulin controls glucose, thyroxine regulates metabolic rate, and vitamin C does not influence calcium homeostasis.