The parathyroid glands sense the serum calcium level, and secrete parathyroid hormone (PTH) if it becomes too low, for example, when dietary calcium intake is inadequate. PTH stimulates the activity of the 1-hydroxylase enzyme in the kidney, resulting in increased production of calcitriol, the biologically active form of vitamin D3. Increased calcitriol production restores normal serum calcium levels in three different ways: 1) by activating the vitamin D-dependent transport system in the small intestine, increasing the absorption of dietary calcium; 2) by increasing the mobilization of calcium from bone into the circulation; and 3) by increasing the reabsorption of calcium by the kidneys. PTH is also required to increase calcium mobilization from bone and calcium reabsorption by the kidneys. However, PTH is not required for the effect of calcitriol on the intestinal absorption of calcium.