Calcium-sensing proteins in the parathyroid glands sense serum calcium levels. In response to slight declines in serum calcium, the parathyroid glands secrete parathyroid hormone (PTH). PTH stimulates the activity of the 1-hydroxylase enzyme in the kidney, resulting in increased production of calcitriol, the biologically active form of vitamin D. Calcitriol activates the vitamin D-dependent transport system in the small intestine, increasing the absorption of dietary calcium and phosphorus. Calcitriol and PTH act on the skeleton to increase the mobilization of calcium and phosphorus into the circulation. In the kidneys, calcitriol and PTH increase calcium reabsorption and increase phosphorus excretion.