What is the role of parathyroid hormone in the body?

Parathyroid hormone (PTH) plays a crucial role in regulating calcium levels in the blood, making it essential for maintaining bone density, muscle function, and overall cellular processes that depend on calcium. When blood calcium levels drop, the parathyroid glands release PTH, which acts primarily on three target sites: the bones, kidneys, and intestines.

In the bones, PTH promotes the release of calcium by stimulating the activity of osteoclasts, which break down bone tissue. In the kidneys, it enhances the reabsorption of calcium and reduces the reabsorption of phosphate, which helps to increase the serum calcium levels. Additionally, PTH stimulates the conversion of vitamin D to its active form, calcitriol, in the kidneys, which in turn increases intestinal absorption of calcium.

This regulation is vital because calcium is involved in numerous physiological processes, including muscle contraction and neurotransmitter release. Therefore, the correct answer reflects the primary function of parathyroid hormone in regulating calcium homeostasis in the body.