Vitamin D metabolism and physiologic effects at target organs Vitamin D metabolism and physiologic effects at target organs. Provitamin D (7-dehydrocholesterol) in the skin is converted to cholecalciferol by ultraviolet (UV) light. Cholecalciferol and ergocalciferol (from plants) are transported to the liver, where they undergo the first step in bioactivation, the hydroxylation at C-25 to 25-hydroxyvitamin D3 [25(OH)D], the major circulating form of vitamin D. The second hydroxylation step, at C-1, occurs in the kidney and results in the hormonally active vitamin D [1,25(OH)2D], also known as calcitriol. This activation step, mediated by 1α-hydroxylase, is under tight regulation by parathyroid hormone (PTH), Ca2+ levels, and vitamin D [1,25(OH)2D]. The activity of 1α-hydroxylase is stimulated by PTH and inhibited by Ca2+ and 1,25(OH)2D. Decreased activity of 1α-hydroxylase favors formation of the inactive form of vitamin D by C-24 hydroxylation. Vitamin D increases bone resorption and formation, increases dietary Ca2+ absorption, facilitates renal Ca2+ reabsorption, and decreases the PTH synthesis by the parathyroid glands. The overall effect of vitamin D is to increase plasma Ca2+ concentrations. Source: Chapter 5. Parathyroid Gland and Ca2+ and PO4- Regulation, Endocrine Physiology, 4e Citation: Molina PE. Endocrine Physiology, 4e; 2013 Available at: https://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/moli4/moli4_c005f009.png&sec=42541007&BookID=507&ChapterSecID=42540505&imagename= Accessed: December 17, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved