Which hormone plays a significant role in regulating calcium levels in the blood?

• A. Calcitonin
• B. Parathyroid hormone
• C. Insulin
• D. Melatonin

Answer: (B)

Parathyroid hormone (PTH) is crucial for maintaining calcium homeostasis in the blood. It is secreted by the parathyroid glands and functions primarily to increase the concentration of calcium in the bloodstream when levels fall below a certain threshold. PTH achieves this by stimulating the release of calcium from bones, enhancing the absorption of calcium in the intestines through activation of vitamin D, and promoting renal reabsorption of calcium, reducing its excretion in urine. This multifaceted approach ensures that calcium levels remain within a narrow, optimal range, which is essential for various physiological processes, including muscle contraction, nerve function, and blood coagulation.

In contrast, calcitonin, while involved in calcium regulation, primarily serves to decrease calcium levels when they are elevated by inhibiting bone resorption and promoting the excretion of calcium by the kidneys. Insulin and melatonin do not play significant roles in calcium metabolism; insulin is primarily involved in glucose metabolism, while melatonin regulates sleep-wake cycles and has no direct impact on calcium levels. Thus, parathyroid hormone is the hormone that plays the dominant role in adjusting and regulating calcium levels in the blood.