What triggers the release of parathyroid hormone (PTH)?

The release of parathyroid hormone (PTH) is primarily triggered by decreased calcium levels in the blood. When calcium levels drop, the parathyroid glands detect this change and respond by secreting PTH. The hormone functions to elevate blood calcium levels through several mechanisms: it stimulates calcium release from bones, increases calcium reabsorption in the kidneys, and enhances the activation of vitamin D, which in turn promotes intestinal absorption of calcium.

In the context of the other options, increased calcium levels would typically inhibit PTH release, while high vitamin D levels influence calcium metabolism but do not directly trigger PTH release. Elevated phosphorus levels can have a complex interaction with calcium and can affect PTH indirectly, but they are not the primary trigger for PTH secretion. Thus, the key factor is the decrease in serum calcium levels.