Which of the following increases plasma calcium levels?

Parathyroid hormone (PTH) plays a crucial role in regulating calcium levels in the plasma. It is secreted by the parathyroid glands in response to low serum calcium levels. PTH acts primarily to increase plasma calcium through several mechanisms: it promotes the release of calcium from bones by stimulating osteoclast activity, increases renal reabsorption of calcium (reducing calcium excretion in urine), and enhances the activation of vitamin D in the kidneys, which in turn increases intestinal absorption of calcium.

This multifaceted approach effectively raises plasma calcium levels to maintain homeostasis and support vital physiological functions. In contrast, other options either do not contribute to increased calcium levels or have opposing effects. For instance, calcitonin works to lower plasma calcium levels, while bone-derived fibroblast growth factor 23 is involved in phosphate regulation and does not raise calcium levels. Insulin primarily regulates glucose metabolism and has no direct role in calcium level regulation. This elucidation of PTH's mechanisms underlines its importance in calcium homeostasis.