Estimation of biologically active intact parathyroid hormone in normal and hyperparathyroid sera by sequential N-terminal immunoextraction and midregion radioimmunoassay.

We report a two-step immunochemical method for estimating serum intact PTH, as defined by immunochemical methods, and its validation by a newly developed osteosarcoma cell adenyl cyclase stimulation assay for PTH bioactivity. The first step involves extraction and concentration of serum PTH moieties with solid phase amino-terminal PTH antibodies; in the second step, the initial PTH immunoextract is analyzed with a sensitive midregion immunoassay. Intact PTH can thus be detected in virtually all normal subjects. Intact PTH levels in our group of primary hyperparathyroid persons average nearly 20 times higher than normal and do not overlap the normal range. Intact PTH is also elevated in chronic renal disease, but less dramatically than in primary hyperparathyroidism. Since total PTH immunoreactivity (intact plus fragments) is much higher in renal disease patients than in persons with primary hyperparathyroidism, serum intact PTH in renal disease apparently comprises a much smaller fraction of total circulating PTH immunoreactivity than in primary hyperparathyroidism. The finding that intact PTH accounts for a large portion of total circulating PTH immunoreactivity in primary hyperparathyroidism is contrary to published reports by us and others. Some of the possible reasons for the differences between our present results and previous reports are examined.