Objective: Primary hyperparathyroidism (pHPT) is associated with hypertension, hyperinsulinaemia, and insulin resistance. The present study investigated the causes of these metabolic disturbances by quantifying insulin sensitivity and glucose effectiveness, and by assessing the time course of beta-cell insulin secretion and hepatic insulin extraction, during a dynamic condition such as after an intravenous glucose load. In addition, we evaluated the possible link between metabolic disorders and high blood pressure.
Subjects: We studied 16 patients with pHPT, before and 12 weeks after parathyroidectomy; eight of these patients were re-evaluated one year after surgery. The control group consisted of 18 healthy volunteers.
Design and measurements: All subjects underwent an oral and a frequently sampled intravenous glucose tolerance test. The data from the intravenous glucose tolerance test were analysed by means of the minimal model technique which yields relevant parameters to comprehend the metabolic status of the single individual.
Results: The glucose intolerance condition was characterized by a severely impaired insulin sensitivity in pHPT (3.2 +/- 0.5 vs 9.5 +/- 1.5 x 10(4)/min/(microU/ml) of control subjects; P < 0.001), as well as by a reduced glucose effectiveness, (0.02 +/- 0.002 vs 0.03 +/- 0.003/min of control subjects; P < 0.04). Total insulin secretion during the 4 hours of the test was almost twofold elevated in comparison to the control subjects (32795 +/- 4769 vs 16864 +/- 1850 pM, P < 0.004) and its basal component significantly correlated with the high blood pressure. Hepatic extraction of insulin was significantly increased in pHPT (85 +/- 2 vs 76 +/- 2%, P < 0.03), possibly as a compensatory mechanism of hypersecretion, which however did not prevent peripheral hyperinsulinaemia in pHPT. Patients with pHPT were divided into two subgroups with normal and impaired glucose tolerance. The patients with impaired glucose tolerance had a significant reduction of first phase insulin response, although their basal and stimulated insulin levels were higher. Tissue insulin sensitivity and glucose effectiveness did not significantly differ between the two subgroups. After surgery, all the biochemical parameters (former hypercalcaemia, hypophosphataemia, elevated parathormone levels) were normalized, insulin sensitivity significantly improved (6 +/- 1 x 10(4)/min/(microU/ml), P < 0.001), whereas glucose effectiveness remained completely unchanged. Basal and stimulated insulin responses were insignificantly lowered after surgery, and hepatic extraction did not change either.
Conclusions: Patients with pHPT exhibited decreased insulin sensitivity and insulin hypersecretion. The latter is only partially ameliorated by increased hepatic insulin extraction. After surgery, although the biochemical abnormalities were fully reversible, the metabolic changes improved only partially.