Immunoassays for the detection of parathyroid hormone

Biologically active parathyroid hormone (PTH) in humans with normal renal function circulates predominantly as an 84 amino acid peptide. PTH fragments of varying length arise either from metabolism of the intact hormone within the parathyroid glands or in peripheral tissues, such as liver, and the resulting carboxyl-terminal peptides are eliminated mainly by glomerular filtration and subsequent tubular degradation. Most of the initially raised anti-PTH antisera were directed against epitopes within the mid- or carboxyl-terminal regions of the hormone. These antibodies were used for the development of conventional, displacement-type radioimmunoassays, but provided only an index of the biologically active PTH(1-84) in the circulation. Subsequently developed immunometric assays use two distinct antibodies, a capture antibody usually directed against a carboxyl-terminal portion of PTH(1-84) and a radio- or enzyme-labeled detection antibody usually directed against the amino-terminal portion of the hormone. Such assays were thought to detect largely, if not exclusively, intact PTH, thus providing the concentration of biologically active hormone in blood, which is especially important for establishing the diagnosis of hyperparathyroidism. However, serum samples from normal subjects and patients with primary or secondary hyperparathyroidism have demonstrated that most immunometric two-site sandwich assays detect, besides PTH(1-84), one or more recently discovered large carboxyl-terminal PTH fragments that lack a portion of the amino-terminal end of the molecule. Some of these amino-terminally truncated PTH molecules [ntPTH(1-84)] exhibit an elution profile on high performance liquid chromatography (HPLC) that is indistinguishable from that of synthetic PTH(784). Such peptides were previously thought to be of minimal if any biological activity, but recent studies have shown that synthetic PTH(7-84) has hypocalcemic properties in vivo and that it inhibits osteoclastic bone resorption and the formation of mature osteoclasts in vitro. It is currently unclear whether important differences in disease states can be revealed by comparing results obtained with older immunometric assays that measure the full-length hormone and ntPTH(1-84) versus newer assays that measure only PTH(1-84). Therefore, whereas most immunometric PTH assay systems are appropriate for the diagnosis of primary hyperparathyroidism, it is possible that immunometric assays designed to detect only PTH(1-84) will be more useful in certain diagnostic studies, for intraoperative PTH monitoring and for assessing the pulsatility of PTH secretion. In addition, the ability to distinguish between the relative concentrations of ntPTH(1-84) versus PTH(1-84) may reveal previously unsuspected roles for the ntPTH(1-84) fragments in the pathophysiology of patients with end-stage renal disease and/or other disorders involving parathyroid hormone.