1. (A) Pathway of vitamin D metabolism
(A) Pathway of vitamin D metabolism. The rate-limiting step in the activation of vitamin D is the 1-α-hydroxylation of 25(OH)D3 to calcitriol and is directly stimulated by PTH and low serum phosphate concentration. (B) Regulation of vitamin D metabolism. PTH is the main stimulator of calcitriol synthesis. The effect of ionized calcium is indirect and mediated by PTH. Calcitriol inhibits its own synthesis and stimulates the production of 24,25(OH)2D3, an inactive vitamin D metabolite.
Source: Disorders of Bone and Mineral Metabolism, Pediatric Practice: Endocrinology, 2e
Citation: Kappy MS, Allen DB, Geffner ME. Pediatric Practice: Endocrinology, 2e; 2013 Available at: Accessed: October 29, 2017
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