Category: biosynthesis
Cholecalciferol (D3, dietary + cutaneous via UV-B on 7-dehydrocholesterol) and ergocalciferol (D2, fungal) → 25-hydroxyvitamin D (calcidiol) via hepatic CYP2R1 — the form measured clinically (reflects body stores; t½ ~3 weeks). 25-OH-D → 1,25-dihydroxyvitamin D (calcitriol, active hormone) via renal CYP27B1 (1α-hydroxylase) under PTH stimulation. Calcitriol → VDR → genomic effects (intestinal Ca/PO4 absorption + bone remodeling + immune modulation). Inactivation by CYP24A1 (24-hydroxylase). CKD-MBD: progressive CYP27B1 deficiency → low 1,25-D → secondary hyperPTH → bone disease + CV calcification. Treatment: calcitriol or paricalcitol (already-active vitamin D analogs) bypassing the failing renal step.