Parathyroid hormone increases serum calcium by which mechanism?

• A. Increasing calcium excretion in urine
• B. Increasing bone resorption and intestinal calcium absorption
• C. Decreasing renal calcium reabsorption
• D. Inhibiting intestinal calcium absorption

Answer: (B)

Parathyroid hormone raises serum calcium through two main actions that work together to restore calcium levels: it promotes calcium release from bone and it increases intestinal calcium absorption, with the kidneys supporting these processes.

In bone, PTH stimulates osteoblasts to produce signals that activate osteoclasts, leading to resorption of bone and release of calcium into the bloodstream. In the kidney, PTH increases calcium reabsorption in the distal tubule, reducing urinary loss, and it stimulates production of active vitamin D (calcitriol) via 1α-hydroxylase. Calcitriol then enhances calcium absorption from the intestine. Together, these effects raise serum calcium.

The option describing increasing bone resorption and intestinal calcium absorption best captures how PTH elevates calcium levels. Choices suggesting more calcium loss in urine, reduced renal reabsorption, or inhibition of intestinal absorption would not increase serum calcium.