A new highly sensitive radioimmunoassay for human calcitonin useful for physiological studies

A highly sensitive and specific radioimmunoassay (RIA) for human calcitonin-like immunoreactivity (CT-LI) has been developed. The assay is performed directly without extraction and may be used for measurements in plasma or serum as well as buffer solutions. The detection limit of the assay is < 0.4 pmol/l, intra- and interassay variations 5 and 10%, respectively when calculated from a control sample within the range of normal concentrations. The recovery of calcitonin (CT) varied between 87 and 118% in the concentration range 10-130 pmol/l. Measurements of CT-LI in sera from 70 normal individuals, showed significantly lower concentrations in women as compared to men, 7.4 pmol/l +/- 2.1 pmol/l and 9.1 pmol/l +/- 2.2 pmol/l, respectively (mean +/- S.D.), P < 0.005. The chromatographic profile of immunoreactive CT in normal serum was investigated by gel chromatography and high pressure liquid chromatography (HPLC) and showed heterogeneity of circulating CT-LI, but with the majority eluting corresponding to the elution position of synthetic human CT. The study of physiological variations in CT levels has so far been hindered by the limitations in assay sensitivity. It is hoped that the significantly improved sensitivity of the presently described assay may contribute towards the elucidation of the physiological functions of CT.