Malignancy hypercalcemia: evaluation of parathyroid function and response to treatment

In 53 patients with malignancy-associated hypercalcemia, we measured serum parathyroid hormone (PTH) with an immunoradiometric assay (IRMA) for intact PTH and a midregion-specific radioimmunoassay (RIA). Values were measured at baseline to detect clinically unrecognized hyperparathyroidism and after treatment with intravenous bisphosphonate to test for a parathyroid response. One patient probably had primary hyperparathyroidism, since his serum PTH values were not appropriately suppressed at baseline and increased markedly during treatment before normocalcemia was achieved. In each of the 51 other evaluable cases, the intact PTH value was suppressed below 25 pg/ml (normal range 10-55 pg/ml), confirming a nonparathyroid hypercalcemia. Midregion PTH values were less fully suppressed but supported the diagnosis in 44 out of 49 cases. The five patients with high midregion PTH values each had renal insufficiency but in four, the intact PTH values were also the highest we observed in these patients. This suggests either a poorly suppressible intact PTH secretion or prolongation of intact PTH half-life by the renal insufficiency. Overall, midregion and intact PTH values were highly correlated (p less than 0.001). Restoration of normocalcemia increased PTH in both assays into or within the normal range, and when posttreatment hypocalcemia occurred, PTH values became elevated in both assays. We conclude the following: (1) both assays are useful for detecting parathyroid hyperfunction but the intact PTH assay is the better diagnostic tool when renal insufficiency is present, (2) hypercalcemia seems to suppress intact PTH secretion more fully than secretion of PTH fragments, and (3) parathyroid glands chronically suppressed by hypercalcemia can increase PTH secretion within 1 or 2 days after the hypercalcemia is corrected.