Patients have normal serum calcium concentrations with a low serum phosphate concentration due to increased urine phosphate excretion. Serum alkaline phosphatase is usually increased. The plasma 1,25-dihydroxyvitamin D concentration may be decreased or inappropriately normal. In the presence of hypophosphataemia, it should be increased. The reason for this failure of increase in 1-hydroxylase activity in the renal tubular cells is unexplained. The basic defect is phosphate depletion due to renal tubular dysfunction, and therapy for this condition involves use of oral phosphate and a vitamin D metabolite. The clinical features of this condition are very similar to those of oncogenic osteomalacia.
