Diagnostic utility of C-terminal parathyrin measurement as compared with measurements of N-terminal parathyrin and calcium in serum.

We compared results obtained from two parathyrin (parathyroid hormone) assays with differing specificities, using sera from 172 normal donors and from 98 patients with disorders of calcium regulation. Intact parathyrin was measured in both assays; the C-parathyrin assay also measured the 53-84 amino acid C-terminal hormone fragment; the N-parathyrin assay also measured the 1-34 N-terminal fragment. The reference interval for the C-parathyrin assay (860-3720 int. units/L; 430-1860 ng/L) was markedly higher than for the N-parathyrin assay (460-1260 int. units/L; 230-630 ng/L), a finding consistent with the longer half-life of C-parathyrin fragments in human circulation. Mean C-parathyrin in primary hyperparathyroid sera--5720 (SD 2760) int. units/L or 2860 (SD 1380) ng/L--clearly exceeded the reference interval and values for sera from patients with non-parathyroid malignancy [1740 (SD 760) int. units/L; 870 (SD 380) ng/L]. Secondary hyperparathyroid patients also had supranormal C-parathyrin values: 6100 (SD 2720) int. units/L; 3050 (SD 1360) ng/L. We found no consistent correlation between parathyrin and serum calcium in any clinical group. The two parathyrin assays showed about equal diagnostic power, but their results could not be used interchangeably in sequential monitoring of patients.