To the Editor.—The serum phosphate concentrations of 4.5 to 5.5 mg/100 ml are within the normal range for serum inorganic phosphate concentrations for infants between 1 and 6 months of age as determined in our laboratory. The change of serum phosphate concentrations with age is well known and is documented by many papers in the literature in addition to the one cited by Dr. Schoen.
The retardation of growth of children with hereditary hypophosphatemia is not necessarily dependent upon delayed treatment and leg deformities following weight bearing as suggested by Dr. Schoen, although such deformities do play a role. In the patients whom we have followed from early infancy, weight bearing deformities were prevented by appropriate treatment started at an early age but retardation of growth was still evident.
It is well known that there is a marked variation among cases of hereditary hypophosphatemia with respect to the severity