How does parathyroid hormone (PTH) affect calcium absorption?

Parathyroid hormone (PTH) plays a crucial role in regulating calcium levels in the blood, and its effects on calcium absorption are closely tied to vitamin D. When PTH is secreted in response to low blood calcium levels, it stimulates the kidneys to reabsorb calcium and also promotes the conversion of vitamin D into its active form, calcitriol. This active form of vitamin D enhances the intestinal absorption of calcium.

The relationship between PTH and vitamin D is essential because PTH does not act independently to increase calcium absorption in the intestines. Instead, its action on the kidneys to promote vitamin D activation is what ultimately facilitates increased calcium absorption from dietary sources in the gut. Therefore, PTH enhances calcium absorption indirectly by necessitating the presence of vitamin D.

This interdependency highlights the importance of both hormones in maintaining calcium homeostasis. In contrast, the other options either misinterpret the role of PTH or state that it does not affect calcium absorption, disregarding the vital connection between PTH, vitamin D, and calcium metabolism.