It does this by acting on all three primary sites of regulation of calcium balance.

First, vitamin D increases the production of several intestinal proteins, including a luminal membrane calcium channel and a high-affinity cytosolic calcium-binding protein (calbindin), that enhance transcellular absorption of calcium.

Second, in the kidney, vitamin D appears to act in a synergistic fashion with PTH to induce active calcium reabsorption in the distal convoluted tubule and connecting tubule by increasing the synthesis of a distinct luminal membrane calcium channel and a cytosolic calcium-binding protein (calbindin).

Third, vitamin D induces resorption of bone, mobilization of calcium, and bone mineralization after an elevation of plasma calcium levels.