Serum 1,25-dihydroxyvitamin D concentration in hypophosphatemic vitamin D-resistant rickets

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Abstract
Fasting serum 1α,25-dihydroxyvitamin D (1,25-(OH)2D) levels were measured in 3 groups of hypophosphatemic vitamin D-resistant rickets (VDRR) patients: those untreated; those treated with vitamin D and phosphate; and those treated with 1,25-(OH)2D3 and phosphate. In the untreated patients, the mean 1,25-(OH)2D level was higher than in our age-matched control group. Except for one at 66 pg/ml, individual values were however within normal limits. Long term vitamin D2 therapy was accompanied by a slight but significant decrease in 1,25-(OH)2D concentrations; nonetheless the levels remained within the normal range. In the third group of patients, the concentration of 1,25-(OH)2D rose to supranormal levels when sampling was done 1–3 hours after administration of the hormone, decreasing rapidly to levels below that of normal subjects when the specimens were collected 12–24 hours later. Our data show that an alteration of the vitamin D activation pathway is unlikely to be part of the pathogenic mechanism underlying the VDRR condition. Calcitriol (1α,25-(OH)2D3) as RocaltrolR in capsules of 0.25 and 0.50 µg was kindly supplied by Dr. Patrick Le Morvan (Hoffmann-La Roche Ltd, Vaudreuil, Que., Canada).