Vitamin D — Pharmacology

Introduction

• A prohormone, not a true vitamin — Under adequate sun exposure vitamin D is fully synthesised in skin, so it is not dietetically essential; its actions are mediated by the vitamin D receptor (VDR), a nuclear receptor of the steroid/thyroid superfamily. KDT classifies it as a hormone — made in the body, blood-borne, activated, receptor-acting and feedback-regulated.
• Two parent forms — "Vitamin D" is a collective name for antirachitic substances: vitamin D₃ = cholecalciferol (skin/animal-derived) and vitamin D₂ = ergocalciferol (plant/fungal ergosterol). Vitamin D₁ is a historic mixture.
• One of three calcaemic hormones — Plasma Ca²⁺ is governed by PTH, calcitriol (active vitamin D) and FGF23 (calcitonin minor), acting on intestine, bone and kidney to handle Ca²⁺ and phosphate.
• D₃ vs D₂ potency — G&G holds D₃ ~10-fold more potent than D₂ (longer D₃ t½; lower D₂-metabolite affinity for vitamin-D-binding protein), overturning the older view; KDT retains "D₂ and D₃ are equally active in man".
• Unitage — 1 µg cholecalciferol = 40 IU; total serum 25-hydroxyvitamin D (25-OHD) is the accepted clinical index of status and of treatment adequacy.