Which of the following substances is primarily responsible for regulating calcium and phosphorus levels in the body?

Parathyroid hormone plays a crucial role in regulating calcium and phosphorus levels in the body. It is secreted by the parathyroid glands and functions to increase the concentration of calcium in the blood when it falls below normal levels. This is achieved through several mechanisms: it promotes the release of calcium from bones, increases the absorption of calcium in the intestines (through its role in activating vitamin D), and reduces the excretion of calcium by the kidneys.

Additionally, parathyroid hormone has an impact on phosphorus levels; it can lower serum phosphate levels by promoting its excretion in the kidneys. This dual action on both calcium and phosphorus is essential for maintaining the body’s mineral balance and is vital for various physiological processes, including bone health, nerve function, and muscle contraction.

In contrast, the other substances listed have different primary roles and do not directly regulate calcium and phosphorus levels. Insulin and glucagon are primarily involved in glucose metabolism, while thyroxine is mainly responsible for regulating metabolism and energy levels.