Vitamin D metabolism: update for the clinician.

Vitamin D3 must undergo two hydroxylation steps before it becomes fully active: 25-hydroxylation in the liver and 1- or 24-hydroxylation in the kidney. Parathyroid hormone, serum phosphate, and serum calcium are important in regulation of renal production of 1,25-dihydroxy vitamin D3 (1,25-[OH]2D3) and 24,25-dihydroxy vitamin D3. An enzyme involved in renal hydroxylation is deficient or defective in patients with chronic renal failure, the Fanconi syndrome, vitamin D-dependent rickets, hypoparathyroidism, and pseudohypoparathyroidism. Altered vitamin D metabolism also occurs in various hepatic diseases, postmenopausal osteoporosis, and anticonvulsant osteomalacia. Recently, 1,25-(OH)2D3 was approved for treatment of renal osteodystrophy. In physiologic doses, it predictably corrects many of the clinical and biochemical abnormalities associated with this disorder.