Which hormone decreases blood calcium levels?

• A. Parathyroid hormone
• B. Vitamin D
• C. Calcitonin
• D. Growth hormone

Answer: (C)

Calcium homeostasis is controlled by hormones that regulate bone breakdown and kidney handling of calcium. Calcitonin lowers blood calcium by inhibiting osteoclasts, which reduces bone resorption and shifts calcium toward bone deposition. It can also promote calcium excretion in the kidneys. This makes it the hormone that decreases blood calcium levels.

Parathyroid hormone raises blood calcium by stimulating bone resorption, increasing renal reabsorption of calcium, and promoting activation of vitamin D to boost gut absorption. Vitamin D itself increases calcium absorption from the gut (and supports bone turnover that can raise calcium). Growth hormone influences growth and has complex effects on calcium metabolism but does not primarily decrease blood calcium.