The Influence of Parathyroid Hormone and Its Fragments on Results from Midregion and C-Terminal Specific Radioimmunoassays^*)

Summary

We compared a midregion (44—68) human parathyroid hormone (hPTH) specific radioimmunoassay (M-RIA) with a C-terminal (65—84) hPTH specific radioimmunoassay (C-RIA). The M-RIA discriminated 21 of 23 patients with primary hyperparathyroidism from 95 normals (normal range: 22.4—106.5 pmol/1). With the C-RIA 12 patients including the 2 patients not discriminated by the M-RIA had immunoreactive PTH (iPTH) values within the normal range of this assay (normal range: undetectable to 88.6 pmol/1). To investigate the reasons for these different abilities of separation, hyperparathyroid sera were subjected to gel-filtration and analyzed using the two assays. Intact PTH appeared to have a major influence on the immunoreactivity of circulating PTH in the M-RIA. In contrast, the C-RIA showed the highest immunoreactivity with midregion -C-terminal PTH fragments. Hyperparathyroid sera not discriminated by the C-RIA but with elevated iPTH in the M-RIA showed decreased amounts of midregion-C-terminal PTH fragments, while intact PTH comprised the highest amount of total circulating PTH immunoreactivity in the M-RIA. From the present results we conclude that the superiority of the M-RIA is due to the determination of intact PTH which is preferable for clinical measurements w^rith relation to the diagnosis of primary hyperparathyroidism.

Key words

Parathyroid hormone - Radioimmunoassay - Primary hyperparathyroidism