What is the primary function of the parathyroid hormone (PTH)?

The primary function of parathyroid hormone (PTH) is to regulate calcium levels in the blood. PTH is secreted by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis. It works by increasing the levels of calcium in the bloodstream through several mechanisms: stimulating the release of calcium from bones, increasing the absorption of calcium in the intestines (often in conjunction with active vitamin D), and reducing the excretion of calcium by the kidneys.

This function is vital because calcium is essential for various physiological processes, including muscle contraction, nerve transmission, and blood clotting. When calcium levels in the blood drop, PTH is released to restore balance, demonstrating its critical role in calcium regulation.