Radioimmunoassays (RIAs) represent the standard methodology for the measurement of parathyroid hormone (PTH). However, there are a number of limitations associated with the measurement of PTH by this technique. Most immunoassys are insensitive and, therefore, cannot detect circulating levels of hormone in normal subjects. Consequently, RIAs cannot distinguish normal from hypoparathyroid levels of PTH. Most RIAs are heterologous systems that often utilize bovine PTH as the radioligand and antisera raised against bovine PTH to measure human PTH.A major factor complicating PTH-RIA techniques is the presence of multiple forms of circulating hormone. A preponderance of midregion/ carboxy-terminal hormone fragments exist in the circulation, but these forms of the peptide are without biological activity and have prolonged half-lives compared to biologically active intact PTH (1-84). The heterogeneity of the hormone contributes to the high noise-to-signal ratio encountered in most PTH-RIAs.The cytochemical bioassay (CBA) for PTH circumvents the problems associated with RIA and represents the most sensitive method currently available for measuring this hormone, but this assay has not gained widespread use because it is labor intensive, has limited sample throughput, and requires expensive instrumentation. Preliminary data suggest that this sensitive assay system can be modified, creating the opportunity to develop a powerful diagnostic assay for commercial purposes that is free of the limitations of the original CBA.National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
