What effect does parathyroid hormone (PTH) have on the kidneys?

Parathyroid hormone (PTH) plays a crucial role in calcium and phosphate homeostasis within the body, particularly through its effects on the kidneys. PTH primarily functions to increase calcium levels in the blood while also influencing phosphate handling.

When PTH is secreted in response to low serum calcium levels, it acts on the kidneys in several ways. It increases the reabsorption of calcium in the renal tubules, which helps to elevate calcium levels in the blood. At the same time, PTH has the effect of decreasing the reabsorption of phosphate, leading to increased phosphate excretion in urine. This is significant because it helps to maintain appropriate levels of calcium and phosphate in the blood.

The combination of increasing calcium reabsorption and decreasing phosphate reabsorption is essential for maintaining the proper balance of these two ions. By achieving this balance, PTH ensures that calcium levels can rise without a corresponding increase in phosphate, which could lead to problems such as calcification of tissues.

In contrast, options that suggest increased phosphate reabsorption or decreased calcium reabsorption misrepresent PTH's actions. Additionally, the inhibition of vitamin D activation does not accurately reflect PTH's role, as PTH actually promotes the activation of vitamin D in