COMPARISON OF COMMERCIALLY AVAILABLE PARATHYROID-HORMONE IMMUNOASSAYS IN THE DIFFERENTIAL-DIAGNOSIS OF HYPER-CALCEMIA DUE TO PRIMARY HYPER-PARATHYROIDISM OR MALIGNANCY

The measurement of immunoreactive parathyroid hormone (iPTH) has become an important part of the differential diagnosis of hypercalcemia. High values in the absence of renal failure are considered diagnostic of primary hyperparathyroidism. It has been suggested that the finding of normal iPTH in the presence of elevated serum calcium concentration also should be considered diagnostic of primary hyperparathyroidism, on the assumption that iPTH should be suppressed if hypercalcemia were due to another cause. However, normal or even somewhat elevated iPTH values have been reported in patients with hypercalcemia due to malignancy. These values have been ascribed to ectopic secretion of parathyroid hormone (PTH). We have undertaken a study to compare commercially available iPTH assays in the different diagnosis of hypercalcemia due to primary hyperparathyroidism or malignancy.