Hyperparathyroidism in patients with X-linked dominant hypophosphatemic rickets--application of the calcium infusion test as an indicator for parathyroidectomy.

Two children with X-linked dominant hypophosphatemic rickets treated with vitamin-D metabolites and phosphate supplementation, for prolonged periods, developed hyperparathyroidism with nephrocalcinosis. Calcium infusion tests were performed in both. In one patient, the initial test was done two weeks after all treatment was stopped. Only moderate decrease in the degree of the phosphaturia was recorded. However, a repeat test, performed after all medications were withheld for another four weeks, showed normal anti-phosphaturic response, and she continued to be treated conservatively. In the other patient, the test was done five weeks after withholding treatment. Failure to suppress the phosphaturia provided strong support for the diagnosis of tertiary hyperparathyroidism. He underwent total parathyroidectomy and the parathyroid histology confirmed the diagnosis. In both, control of parathyroid activity stopped the deterioration in kidney function and improved the response of the basic disorder to treatment. It is concluded that in patients with X-linked dominant hypophosphatemic rickets, the calcium infusion test is useful for the differentiation between secondary-reversible and tertiary-irreversible hyperparathyroidism. To avoid continued stimulation of the parathyroid glands by phosphate administration, we recommend that such calcium infusion test be performed and interpreted after at least six weeks have elapsed without phosphate or vitamin-D administration.