Radioimmunoassay of human parathyroid hormone in serum

A new radioimmunoassay for human parathyroid hormone (PTH) in serum, which can measure the hormone present in 94% of the normal sera tested, is described. It is based on the ability of human PTH to compete with (131)I-labeled bovine PTH for binding to an antiserum directed against porcine PTH. This antiserum distinguishes between human PTH extracted from parathyroid adenomata and that present in hyperparathyroid sera. Evidence is given to suggest that this is due to immunochemical changes in the hormone extracted from adenomata and not to immunochemical heterogeneity of the hormone present in serum. Physiologic data supporting the validity and specificity of the assay are presented. Induced episodes of hypercalcemia and hypocalcemia resulted in appropriate responses in serum immunoreactive PTH (IPTH) in normal subjects and in patients with Paget's disease of bone. In normals, there was a progressive increase in serum IPTH in the late afternoon and evening, suggesting a diurnal secretory rhythm. A negative correlation was found between the serum calcium and serum IPTH over the normal range of serum calcium values; a positive correlation was found between these variables in patients with primary hyperparathyroidism. There was apparent overlap between serum IPTH values in normal subjects and patients with primary hyperparathyroidism, but formal discriminate analysis of values for serum calcium and IPTH demonstrated separation of these two groups, without overlap.