Decreased plasma calcitonin response to a calcium clamp in primary hyperparathyroidism

In order preoperatively to evaluate the calcium induced plasma calcitonin (CT) response in patients with mild primary hyperparathyroidism (PHPT), 7 postmenopausal (PM) females with PHPT and 14 PM healthy subjects underwent a 2 h calcium infusion. The infusions were performed by means of the 'calcium clamp' technique previously described by us in order to obtain an identical calcium stimulus in all subjects. Immunoreactive calcitonin (iCT) was determined by means of a sensitive RIA utilizing rabbit antibodies directed against the carboxy-terminal amino acid sequence of the CT molecule. All patients with PHPT had normal basal plasma iCT levels: 10 +/- 5 pg-eq/ml (mean +/- SD) as a group and not different from the control group: 13 +/- 5 pg-eq/ml. In the control group significantly increased plasma iCT was seen at 30, 60, 90 and 120 min in contrast to that of the PHPT group where a blunted response was observed. The results indicate a diminished CT reserve in these patients which may imply an impaired defence against the PTH mediated bone resorption seen in patients with PHPT.