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Micronutrient Information Center

The Vitamin D Endocrine System

The parathyroid glands sense the serum calcium level, and secrete parathyroid hormone (PTH) if it becomes too low. When dietary calcium intake is inadequate, PTH stimulates the activity of the 25-hydroxyvitamin D-1alpha-hydroxylase enzyme in the kidney, resulting in increased production of 1,25-dihydroxyvitamin D (1,25 (OH)2D; calcitriol), the biologically active form of vitamin D. Increased 1,25-dihydroxyvitamin D production restores normal serum calcium levels in three different ways: (1) by activating the vitamin D-dependent transport system in the small intestine, which increases 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 1,25-dihydroxyvitamin D on the intestinal absorption of calcium.