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Calcium metabolism in multiple myeloma.

45Calcium metabolism and circulating levels of iPTH (N-terminal fragment) and iCT were investigated in normocalcemic patients with multiple myeloma (12) in an attempt to ascertain early changes in calcium metabolism, which may occur before hypercalcemia develops. The same parameters were also investigated in patients with senile osteoporosis (11), corticosteroid-induced osteoporosis (6), Paget's disease (6) and controls without bone disorders (13). In the myeloma group, the exchangeable calcium pool (7,678 +/- 321 mg) was significantly higher (P less than 0.01) than in the control group (4,405 +/- 374 mg), the senile osteoporosis group (4,108 +/- 407 mg) and the corticosteroid-induced osteoporosis group (3,015 +/- 161 mg) and nearly as high as in the Paget's disease group (8,876 +/- 1,173 mg). Calcium pool turnover rate was higher in the myeloma group than in controls, as were bone anabolism and bone catabolism, although differences were not statistically significant. There were no statistically significant differences among the groups in the plasma iPTH or iCT, although the mean value of the latter was higher in the myeloma group than in controls (86 +/- 24 vs. 47 +/- 13 pg/ml). These data suggest that an increase in the exchangeable calcium pool and in turnover rate may occur early in the course of multiple myeloma, preceding the development of hypercalcemia. The role of some homeostatic mechanisms in maintaining normal plasma calcium levels in multiple myeloma despite increased bone calcium resorption is discussed.