Which of the following hormones lowers blood calcium levels?

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

Answer: (A)

Calcitonin is a hormone produced by the thyroid gland that plays a critical role in regulating calcium levels in the blood. Its primary function is to lower blood calcium levels by inhibiting bone resorption—the process by which osteoclasts break down bone tissue and release calcium into the bloodstream. Calcitonin also promotes the incorporation of calcium into bones by stimulating osteoblasts, which are the cells responsible for bone formation.

In contrast, parathyroid hormone increases blood calcium levels by stimulating osteoclast activity and enhancing calcium reabsorption in the kidneys. Growth hormone primarily influences growth and metabolism but does not directly regulate calcium levels. Cortisol has various effects on metabolism and the immune system but is not involved in the direct regulation of calcium levels like calcitonin is.

Thus, calcitonin is specifically identified as the hormone that functions to lower blood calcium levels, making it the correct answer in this context.