Anomalous effects of hormone fragments on the measurement of parathyroid hormone by radioimmunoassay.

One of the basic assumptions underlying the use of radioimmunoassay and other competitive protein-binding assays is the homogeneity of the antigen or ligand. This assumption is not valid for the measurement of parathormone (PTH) because of the presence of fragments. Hence, there is a potential for errors and high variability in the measurement of parathyroid hormone (PTH) by radioimmunoassay. Even though region-specific radioimmunological and immunoradiometric assays for PTH measurement can overcome some of the difficulties caused by the presence of hormone fragments, the possibility for serious measurement errors still remains. We therefore examined experimentally and by modeling the impact of fragments on the estimation of the concentration of a highly purified intact bovine parathyroid hormone by radioimmunoassay. Our experimental results show that the mere presence of fragments can lead to a significant underestimation or overestimation of the amount of the intact hormone. The results have been simulated by a model in which fragments bind to the antibodies, thus competing with the intact hormone, and to the intact hormone as well, thereby reducing the amount of free intact hormone in competition with the radioligand. This work indicates that it may be preferable to consider alternative methods, other than competitive protein-binding assays, for the measurement of secreted PTH.