The regulation of vitamin-D dosage in resistant rickets (familial or essential hypophosphatemia) is best accomplished by measuring the twenty-four-hour urinary calcium output against the curve of normal excretion. For such comparison the subject's weight and a fairly close estimate of calcium intake are also necessary. Qualitative tests for urine calcium are of no real value in determining the child's status with respect to vitamin-D therapy.

Urinary calcium values before treatment are usually three standard deviations or more below the normal mean excretion value. With high doses of vitamin D, urinary calcium rises toward the mean, the rate of rise paralleling the size of the dose. Maintenance of urine calcium at values approximating the normal mean will promote healing of clinical rickets and avoid hypervitaminosis D. A rise of urinary calcium to approximately three standard deviations above the mean, or higher, means hypervitaminosis, and the dosage should be decreased sharply. The abnormal rise in urinary calcium from overtreatment precedes a rise in serum calcium by days or weeks, except after operative treatment when the child will be thrown into acute hypervitaminosis D unless the vitamin is discontinued for a time starting before operation.