Which hormone decreases blood calcium levels?

• A. To increase calcium levels in the blood
• B. To regulate glucose metabolism
• C. To decrease calcium levels in the blood.
• D. To stimulate parathyroid hormone release

Answer: (C)

The main idea is calcium homeostasis: calcitonin lowers blood calcium, while parathyroid hormone raises it. Calcitonin, released by thyroid C cells when calcium is high, acts to decrease serum calcium by two main actions: it inhibits osteoclasts, reducing bone resorption and calcium release from bone, and it promotes calcium excretion by the kidneys. Parathyroid hormone does the opposite—stimulating bone breakdown, increasing intestinal calcium absorption via vitamin D activation, and enhancing renal calcium reabsorption—so it raises blood calcium. Therefore, the statement describing a decrease in blood calcium reflects calcitonin’s role, making it the correct concept. The other options point to increasing calcium or to functions outside calcium control (like glucose metabolism), which would not lower calcium.