The Tubular Reabsorption of Phosphate in the Differential Diagnosis of Metabolic Bone Disease

The percentage of filtered phosphorus that is reabsorbed by the renal tubules (TRP) was measured in normal subjects and in patients suspected of having metabolic bone disease. When dietary intake of phosphorus before the test was not controlled, the TRP ranged between 77 and 93 per cent in normal subjects and patients without demonstrable disturbances of calcium and phosphorus metabolism. No correlation could be established between the serum phosphorus concentration and the TRP. If the diet contained adequate amounts of phosphorus, the TRP was low in all of the cases of primary hyperparathyroidism that were studied; after phosphorus deprivation, the TRP was often in the normal range. These observations are considered to be consistent with the view that parathyroid hormone lowers the maximal tubular capacity for phosphate reabsorption. The TRP was consistently low in patients with malignant neoplasms complicated by hypercalcemia. The chief usefulness of the TRP test in clinical diagnosis lies in the early diagnosis of hyperparathyroidism when only slight abnormalities of the serum calcium and phosphorus concentration can be detected.