Name: Vitamin D3/cholecalciferol (Delta-D)
Class: Vitamin
Mechanism: Increases intestinal absorption of calcium and phosphate. Stimulates bone resorption by facilitating effects of PTH. Stimulates renal reabsorption of calcium and phosphate. Net result = calcium, phosphate.
Absorption: Oral usu ® adequate absorption. Bile is essential for absorption.
Dist.: Stored in fat and muscle. Tightly bound to vitamin D-binding protein.
Metabolism.: Requires hydroxylation in liver and kidney for full activity.
Excretion, t½: 1° = bile. Weeks.
Toxicity/S.E.s: Excess accumulation in fat/muscle, hypervitaminosis D, hypercalcemia. Drug interactions—phenytoin and phenobarbital reduce sensitivity to vit. D and/or increase rate of inactivation of calcitriol.
Utility: Treat nutritional rickets—prophylactic dose = 400 U/d, fully developed rickets = 3,000-4,000 U/d. Ameliorates Type I vit. D-dependent rickets (4,000 U/d). Treat osteodystrophy 2° to malabsorption of vit. D and calcium—25-50,000 U 3x/wk + calcium. Supplement to estrogen and calcium to treat postmenopausal osteoporosis. Treat hypoparathyroidism (25-100,000 U 3x/wk + oral calcium); use calcitriol or dihydrotachysterol for faster action or if hypervit. D develops.
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