Profound hypophosphatemia and isolated hyperphosphaturia in two cases of multiple myeloma

We describe the development of severe hypophosphatemia and urinary phosphate wasting in two patients with multiple myeloma. In both cases, the serum phosphorus was repeatedly less than 1.0 mg/dL despite vigorous replacement, and the calculated fractional excretion of urinary phosphorus was greater than 100%. Neither patient demonstrated other tubular defects typical of Fanconi's syndrome. With treatment of the myeloma, both patients achieved normalization of the serum phosphorus and no longer required phosphorus supplementation. We believe that multiple myeloma should be considered in the differential diagnosis in patients with profound hypophosphatemia, urinary phosphate wasting, and otherwise intact tubular function.