Calcium and phosphate metabolism in tumoral calcinosis

We have recently seen a patient with tumoral calcinosis, a syndrome comprising hyperphosphatemia, normocalcemia, normal glomerular filtration rate (GFR), and extensive periarticular calcific masses. Parathyroid hormone (PTH) deficiency or target organ resistance was ruled out by demonstration of normal serum PTH and urinary 3'5'cyclic AMP excretion and normal response to exogenous PTH and to endogenous stimulation by ethylenediaminetetraacetate. An intrinsic proximal tubular defect allowing enhanced renal PO4 reabsorption was probably present because there was no phosphaturic response to acetazolamine and renal PO4 threshold remained abnormally elevated even after PTH infusion. We then studied the mechanism by which serum calcium level is maintained in the normal range despite hyperphosphatemia and absence of secondary hyperparathyroidism. Normal 1,25-(OH)2 vitamin D was found, suggesting normal gastrointestinal calcium absorption. This, combined with markedly reduced urinary calcium excretion, perhaps a direct effect of hyperphosphatemia, may maintain calcium balance and prevent secondary hyperparathyroidism. A rise in urinary cyclic AMP excretion after furosemide-induced calciuria supports this hypothesis.