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

The primary function of parathyroid hormone (PTH) is to increase blood calcium levels. PTH is secreted by the parathyroid glands in response to low blood calcium levels. It achieves this by stimulating several key processes: enhancing the release of calcium from bones, increasing the reabsorption of calcium in the kidneys, and promoting the activation of vitamin D, which in turn increases calcium absorption from the intestines.

When PTH is released, it acts rapidly to ensure that calcium levels in the bloodstream are restored to normal. This regulation of calcium is crucial because calcium plays a vital role in various physiological processes, including muscle contraction, blood clotting, and nerve conduction.

The other options pertain to functions that are not related to PTH. Lowering blood glucose levels is primarily the role of insulin, produced by the pancreas. Stimulating insulin production is also a function solely attributed to the pancreas in response to elevated blood sugar. Regulation of metabolism involves hormones such as thyroid hormones and is not directly influenced by PTH. Thus, the correct answer highlights the specific and critical role of PTH in maintaining calcium homeostasis in the body.