Autosomal hypophosphataemic bone disease responds to 1,25-(OH)2D3

We diagnosed non X-linked hypophosphataemic bone disease in a 38-month-old girl. Findings included: genu varum, shortened stature, fasting hypophosphataemia (2.3-2.5 mg/100 ml; 0.74-0.81 mmol/l), diminished theoretical renal threshold for phosphate (TmP/GFR), and osteomalacia without rickets. One patient (the father) had fasting hypophosphataemia (2.3-2.7 mg/100 ml; 0.74-0.87 mmol/l) and low TmP/GFR without osteomalacia or shortened stature. Treatment of the girl with 1,25-(OH)2D3 (1 microgram a day) raised the level of serum phosphorus, improved tubular reabsorption of phosphate, and healed the bone deformity; this combination of responses is not present in X-linked hypophosphataemia. There was no correction of hypophosphataemia or TmP/GFR with 1,25-(OH)2D3 treatment (1-3 micrograms a day) in the father.

X-linked hypophosphatemia: effect of calcitriol on renal handling of phosphate, serum phosphate, and bone mineralization

Eleven patients with the Mendelian phenotype of x-linked hypophosphatemia (XLH) were treated with calcitriol [1,25-(OH)2D3] and phosphate. Ten patients had received prior treatment with ergocalciferol and phosphate. Five subjects were prepubertal and six were postpubertal. Response to calcitriol was measured under nonfasting and overnight fasting protocols. Bone biopsies were obtained before and after treatment. Calcitriol (mean dose, 30 ng/kg. day) 1) raised serum phosphorus uniformly in prepubertal patients but in only two of six postpubertal subjects; 2) did not change the theoretical renal phosphate threshold in the total patient group and thus had no effect on the primary transport defect in XLH; 3) improved trabecular bone mineralization in the total patient group, as determined by bone histomorphometry. The beneficial effect on extracellular phosphorus homeostasis was attributed to improved intestinal absorption of phosphorus; improvement in bone mineralization may reflect an additional effect of 1,25-(OH)2D3 on bone itself in XLH. Mild transient hypercalcemia occurred during 0.6% of 3545 treatment days and was readily controlled by adjusting the dosage of 1,25-(OH)2D3.