Improved diagnosis of primary hyperparathyroidism by defining the inverse relationship between serum immunoreactive parathyroid hormone and calcium

Serum concentrations of immunoreactive parathyroid hormone (iPTH) measured with a mid-region specific radioimmunoassay and total calcium were correlated in 300 healthy subjects and 158 patients with surgically verified primary hyperparathyroidism (HPT). All the healthy individuals could be separated from the patients by a monoexponential declining curve in which iPTH at concentrations of 0.60 micrograms/l and 0.33 micrograms/l corresponded to calcium concentrations of 2.20 mmol/l and 2.60 mmol/l, respectively. In 22 patients more than one sample was analysed and serum iPTH and calcium were inversely correlated. In contrast, three patients with parathyroid carcinoma showed no reciprocal fluctuations between serum iPTH and calcium. Of 75 patients with hypercalcaemia due to malignant diseases (metastatic mammary carcinoma, bronchial carcinoma, renal carcinoma, myelomatosis), 62 had a normal iPTH/calcium relationship. Two patients with myelomatosis had a temporary elevation of serum iPTH and calcium due to renal impairment. One patient with bronchial carcinoma probably had ectopic production of iPTH. The remaining 10 patients (six mammary carcinomas and four bronchial carcinomas) were found in the pathological iPTH/calcium range. In conclusion, we have demonstrated that an inverse relationship exists between serum iPTH and calcium in patients with non-malignant, primary HPT. Evaluation of iPTH and calcium in the same serum sample gave a correct diagnosis in more than 90% of patients with primary HPT.