Posttransplantation hyperparathyroidism: Demonstration of retained control of parathyroid function by plasma ionized calcium

Persistent hyperparathyroidism is a common complication of successful renal transplantation. This study sought to address several generally held assumptions concerning this endocrine disorder, i.e., it represents an autonomous condition that involutes with time. Eight normal subjects and 13 renal transplant patients with hyperparathyroidism were studied. Ten blood samples for simultaneous measurement of serum ionized calcium (Ca⁺⁺) and parathyroid hormone (PTH) were obtained from each subject in the course of a 4-hr Ca⁺⁺ infusion (15 mg/kg) and 2-hr EDTA infusion (50 mg/kg). The results demonstrate that a significant (P < 0.001) inverse logarithmic (sigmoidal) relationship exists between serum Ca⁺⁺ and peripheral PTH levels. The slopes of the relationship were similar, but at any level of serum Ca⁺⁺, the PTH levels were significantly (P < 0.001) higher in the hyperparathyroid transplant recipients. We conclude that posttransplant hyperparathyroidism is not an autonomous condition as the parathyroid gland is equally responsive to acute changes in serum Ca⁺⁺. The excessive levels of basal PTH secretion most likely reflect increased gland mass and not altered sensitivity of the parathyroid cell or metabolism of PTH. Proper management of this disorder requires maintenance of the serum Ca⁺⁺ at some optimal concentration to insure suppression of the gland. If the level of hypercalcemia is unacceptable, surgical intervention with reduction of gland mass is indicated.