Vitamin D metabolism and osteomalacia in cystic fibrosis

A 25-yr-old black man with cystic fibrosis and cirrhosis developed symptoms of osteomalacia and hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and low circulating 25-hydroxyvitamin D (25-OHD). Serum 1,25-dihydroxyvitamin D (1,25-[OH]₂D) was within the normal range. Iliac crest bone biopsy confirmed the diagnosis of osteomalacia. Oral administration of 50,000 IU of vitamin D₂ failed to relieve symptoms or raise serum 25-OHD levels to normal. Intramuscular vitamin D₂, 10,000 IU every 8-12 week, improved symptoms, raised serum 25-OHD to normal, and increased circulating 1,25-[OH]₂D to values five times normal. Over the next 10 mo circulating 1,25-[OH]₂D remained elevated despite normalization of serum calcium, phosphorus, and parathyroid hormone. Repeat bone biopsy 1 yr after parenteral vitamin D showed healing of the osteomalacia. Malabsorption of vitamin D appears secondary to profound steatorrhea due to pancreatic insufficiency and secondary biliary cirrhosis. Although extensive hepatocellular disease was present, hepatic conversion of vitamin D to 25-OHD was intact. Both high and low circulating 1,25-[OH]₂D levels during active osteomalacia have been reported; initially, the level was in the normal range and higher values in this patient occurred with repletion of 25-OHD substrate. This study shows that symptomatic osteomalacia may be a major manifestation of cystic fibrosis in those patients surviving into adulthood. Measurements of serum 25-OHD in cystic fibrosis patients may identify those who should receive supplemental vitamin D.