Which hormone is primarily involved in the regulation of calcium metabolism in bones?

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

Answer: (C)

Parathyroid hormone (PTH) plays a pivotal role in regulating calcium metabolism in bones. It is secreted by the parathyroid glands in response to low serum calcium levels. The primary function of PTH is to increase the concentration of calcium in the blood by acting on bones, kidneys, and the gastrointestinal tract.

In bones, PTH stimulates the activity of osteoclasts, the cells responsible for bone resorption. As these cells break down bone tissue, calcium is released into the bloodstream, thereby increasing serum calcium levels. Additionally, PTH influences the kidneys to retain calcium and enhance the absorption of calcium from the diet in the intestines by promoting the synthesis of active vitamin D.

While calcitonin also affects calcium levels by lowering serum calcium and inhibiting osteoclast activity, its primary function is not the regulation of calcium in bones, but rather a counterbalance to PTH. Estrogen is more involved in bone density maintenance and can influence calcium metabolism indirectly, while cortisol primarily has an impact on bone density regulation through its antagonistic effects. Thus, the direct role of parathyroid hormone in enhancing calcium levels makes it the key hormone in bone calcium metabolism.