The effect of an oral calcium load on plasma ionized calcium and parathyroid hormone concentrations in osteoporotic postmenopausal women.

Plasma ionized calcium (IC) and parathyroid hormone (PTH) concentrations were measured in 31 osteoporotic postmenopausal women at hourly intervals for 5 hours after a 1 g oral calcium load. Fifteen subjects had normal radiocalcium absorption and 16 subjects were malabsorbers of calcium. IC rose and PTH fell after the calcium load in both groups with a plateau at 3-4 hours, and the rise in IC was greater (P less than 0.01) in the normal absorbers. There was a nonsignificant trend for the fall in PTH to be greater in the normal absorbers. In the group as a whole the mean increase in IC (above baseline) at 4 hours was directly related to calcium absorption (P less than 0.025) and the mean change in PTH was inversely related to calcium absorption (P less than 0.05). These results demonstrate that in subjects with postmenopausal osteoporosis the responses of IC and PTH to an oral calcium load are a function of calcium absorptive status.