Diminished acute response of osteoclasts to calcium load in thyroidectomized patients

To elucidate the role of endogenous calcitonin (CT) in the regulation of bone resorption, we evaluated the acute effects of an intravenous calcium load in nine patients after total thyroidectomy (aged 29.2 +/- 8 years) compared with nine healthy subjects. After overnight fasting, intravenous infusions of elemental calcium 1.7 mg/kg body weight were given over a 10-minute period. Blood samples for measurements of serum ionized calcium (S-iCa), plasma intact CT, parathyroid hormone (PTH), and plasma type I collagen cross-linked C-terminal telopeptide (beta-CTX) were obtained 3 minutes before and at 13, 30, 60, 90, and 150 minutes after the start of the infusion. At baseline, parameters of calcium and bone metabolism were similar in both groups. A similar increase in S-iCa and decrease in plasma PTH levels were observed in both groups. However, the plasma CT increased significantly by 13 minutes (P < 0.05) and beta-CTX decreased significantly as early as 30 minutes (P < 0.05) (decrease by 36% as compared with the baseline) only in the group consisting of healthy individuals. In the thyroidectomized group, the plasma beta-CTX did not decrease significantly during the first 60 minutes (decrease by only 8% as compared with the baseline) and response to the calcium load was significantly diminished throughout the study period as compared with that of the healthy subjects (P < 0.01). In conclusion, the results indicate that the increased CT secretion is responsible for the rapid initial decrease in the bone resorption following an acute intravenous calcium load.