Clinical Utility of an Immunoradiometric Assay for Parathyroid Hormone (1–84) in Primary Hyperparathyroidism

The reliable diagnosis of primary hyperparathyroidism depends on the measurement of PTH. The PTH assays in widespread use measure not only the hormone but also hormone fragments, thus limiting the clinical utility of the assays. A new immunoradiometric assay (IRMA) using an antigenic determinant at the extreme amino-terminal of the PTH molecule detects only full-length PTH (1–84). We compared three PTH assays and determined the presence of PTH (1–84) and PTH fragments in serum and parathyroid adenomas of patients with primary hyperparathyroidism. We studied 56 patients with primary hyperparathyroidism. PTH levels were increased in 63% using the midmolecule RIA; in 73% in the “intact” IRMA; and in 96% in the PTH (1–84)-IRMA. The PTH (1–84)-IRMA correlated with the other assays (midmolecule RIA R = +0.736; P < 0.0001; “intact”-IRMA R = +0.951; P < 0.0001) and indices of disease activity (serum calcium R = +0.511, P < 0.0001; alkaline phosphatase R = +0.489, P = 0.001; and radius bone density R = −0.366, P < 0.01). In 21 consecutive patients undergoing parathyroidectomy, 18 had parathyroid adenomas. Intact PTH was higher than PTH (1–84)-IRMA in both serum and glandular homogenates from these patients. Similar proportions of PTH (1–84) and hormone fragments were found in both adenomas [66 ± 3% of “intact” PTH-reflected PTH (1–84) and sera (73 ± 2% of “intact” PTH reflected PTH (1–84)]. We conclude that the PTH (1–84)-IRMA offers improved diagnostic sensitivity in patients with primary hyperparathyroidism than other currently available assays. This study also provides evidence that both PTH (1–84) and PTH fragments are produced in parathyroid adenomas and that peripheral metabolism of hormone and fragment does not alter the proportion of bioactive hormone.