Sensitive homologous radioimmunoassay for human parathyroid hormone to diagnose hypoparathyroid conditions

A sensitive radioimmunoassay for human parathyroid hormone (hPTH) to diagnose hypoparathyroid conditions is described. The antiserum was raised in goats against extracted hPTH. Synthetic human PTH fragment (53-84) is used as a standard and ¹²⁵I-Tyr⁵²hPTH (53-84) as a tracer. After a two-step incubation (24 h + 24 h) at 4°C, the bound and free fractions are separated by a mixture of second antibody and polyethylene glycol solution. The detection limit for hPTH (53-84) is 2 pmol/L hPTH (53-84) (2 mol/tube). The PTH level in 36 healthy subjects was 5-12 pmol/L. Of 14 patients with hypoparathyroidism 11 patients had PTH concentrations below normal, two patients had levels on the lower limit of the normal range (5 pmol/L). The concentration of one patient with pseudohypoparathyroidism was markedly elevated. This assay is suitable for detecting low PTH levels and for studying changes of PTH concentration within the normal range.