Hypercalcitoninemia Associated with Return of Serum Calcium Concentration toward Normal in Chronically Parathyroidectomized Rats*

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Abstract
In chronically parathyroidectomized male Fischer and Holtzman rats, we found a progressive rise in the levels of serum immunoreactive calcitonin (CT) despite hypocalcemia. The rise was preceded by an increase in the thyroidal CT concentration (determined by RIA) and hyperplasia of C cells (determined by the immunoperoxidase bridge technique). In association with the rise in serum CT, we elso observed a return of serum Ca toward normal levels, which the evidence suggests was not due to compensatory regeneration of parathyroid remnants after incomplete parathyroidectomy. It also is unlikely that ectopic parathyroid tissue was responsible for the return of serum Ca toward normal. Since the increase in serum CT after parathyroidectomy coincided with the return of serum Ca toward normal, we postulate that a gradual rise in serum Ca in chronically parathyroidectomized rats might stimulate the “CT-saturated” thyroid glands to release CT, thus leading to a progressive increase in serum CT. Alternatively, it is possible that the increase in serum CT after parathyroidectomy may be due in part simply to nonspecific leakage of CT from the “CT-saturated” thyroid glands.