Medullary nephrocalcinosis is usually associated with hypercalcuria and often with hypercalcemia. Primary hyperparathyroidism and distal renal tubular acidosis account for over 60% of these cases. The remaining 40% of cases are due to a variety of other causes of hypercalciuria, including milk-alkali syndrome, Cushing's syndrome, bony metastases, hyperthyroidism, hypothyroidism, sarcoidosis, furosemide therapy, and in this case, vitamin D and phosphorus therapy in a case of hypophosphatemic rickets.

X-linked hypophosphatemic rickets is a congenital metabolic bone disease produced by a urinary phosphate leak. Vitamin D and oral phosphate supplementation therapy has proven useful in the treatment of the metabolic bone disease. Medullary nephrocalcinosis may occur in 33 to 50% of patients with x-linked hypophosphatemic rickets who have been treated with vitamin D and phosphate therapy, whereas this abnormality is seen in untreated patients. The medullary nephrocalcinosis is due to hypercalcemia and hypercalciuria which occurs in these patients on vitamin D and phosphate therapy. It alone has not been associated with clinically impaired renal function. However, if secondary hyperparathyroidism develops, both increased medullary and cortical echogenicity may be seen associated with impaired renal function. In this patient, there was clinical evidence for secondary hyperparathyroidism, which was felt to be responsible for the decreasing renal function even though the renal cortical echogenicity was not increased on ultrasound.