Which hormone regulates normal blood calcium levels?

• A. thyroxine
• B. estriol
• C. parathyroid hormone
• D. growth hormone

Answer: (C)

Calcium homeostasis is controlled mainly by a hormone that detects low serum calcium and mobilizes stores while promoting calcium reabsorption and intestinal absorption. The parathyroid hormone does exactly that: when calcium falls, it is secreted by the parathyroid glands and stimulates bone resorption to release calcium, increases renal reabsorption of calcium to reduce urinary loss, and promotes formation of active vitamin D in the kidney, which enhances intestinal calcium absorption. This coordinated action raises blood calcium toward normal. The other hormones listed regulate different physiological processes: thyroxine mainly sets metabolic rate, estriol (estrogen) influences bone density over time but isn't the rapid regulator of calcium, and growth hormone has broad growth and metabolic effects but does not primarily regulate calcium levels.