From which locations does PTH primarily regulate calcium levels?

Parathyroid hormone (PTH) plays a critical role in maintaining calcium homeostasis within the body, primarily by acting on three key sites: the bones, intestines, and renal tubules. Understanding the functions of PTH at these locations is vital in appreciating how the hormone contributes to calcium regulation.

In the bones, PTH stimulates osteoclast activity, leading to the release of calcium stored in the bone matrix into the bloodstream. This process increases serum calcium levels quickly in response to low calcium concentrations.

The intestines are influenced indirectly by PTH. While PTH itself does not act directly on the intestines, it enhances the absorption of calcium from the gastrointestinal tract by promoting the activation of vitamin D (specifically calcitriol), which enhances intestinal calcium uptake.

In the renal tubules, PTH plays a direct role by promoting the reabsorption of calcium and excreting phosphate. This action helps to conserve calcium ions while also managing phosphate levels in the body.

This triad of influence—bones, intestines, and renal tubules—ensures that serum calcium levels are maintained within a narrow physiological range, demonstrating the comprehensive regulatory role of PTH.