What is the primary function of parathyroid hormone?

The primary function of parathyroid hormone (PTH) is to increase calcium availability in the blood. This hormone is secreted by the parathyroid glands, which are located near the thyroid gland in the neck. When blood calcium levels drop below normal, PTH is released into the bloodstream. It acts on various target tissues, including bones, kidneys, and the intestines, to increase calcium levels.

In bones, PTH stimulates the activity of osteoclasts, cells responsible for breaking down bone tissue, leading to the release of calcium into the bloodstream. In the kidneys, PTH promotes the reabsorption of calcium, reducing the amount lost in urine, and it also promotes the activation of vitamin D, which enhances calcium absorption in the intestines. The net effect of these actions is to ensure that calcium levels in the blood are maintained within a narrow range, which is critical for various bodily functions such as muscle contraction and nerve transmission.

This regulation of blood calcium is vital for maintaining many physiological processes, whereas the other options do not align with the primary functions of PTH.