1,25-dihydroxyvitamin D suppresses circulating levels of parathyroid hormone in a patient with primary hyperparathyroidism and coexistent sarcoidosis

CONTEXT: PTH is excessively secreted to develop hypercalcemia and accelerate bone turnover in patients with primary hyperparathyroidism. PTH stimulates the production of 1,25-dihydroxyvitamin D [1,25(OH)2D] that in turn suppresses the synthesis of PTH in parathyroid cells. OBJECTIVE: The objective of the study was to clarify whether 1,25(OH)2D indeed inhibits circulating levels of PTH and influences bone turnover, even in a patient with primary hyperparathyroidism. DESIGN, SETTING, AND PATIENT: We evaluated PTH levels in a patient with primary hyperparathyroidism and coexistent sarcoidosis whose serum 1,25(OH)2D levels were independent of PTH. INTERVENTIONS AND MAIN OUTCOME MEASURES: The present case was treated with prednisolone before and after surgical resection of parathyroid adenoma, and Ca-regulating hormones and bone markers were measured. RESULTS: Serum Ca and PTH levels significantly decreased after parathyroid surgery, whereas serum 1,25(OH)2D levels remained high. Prednisolone administration promptly decreased serum 1,25(OH)2D levels and reciprocally increased PTH levels despite consistent serum Ca levels either before or after surgery. PTH levels were negatively correlated with serum 1,25(OH)2D levels before and after surgery. Urine N-telopeptides, serum osteocalcin, and bone-type alkaline phosphatase all decreased to physiological ranges after parathyroid surgery. CONCLUSIONS: These results suggest that 1,25(OH)2D indeed inhibits the production of PTH not to exacerbate hypercalcemia in a patient with primary hyperparathyroidism. Furthermore, PTH but not 1,25(OH)2D may primarily be involved in the stimulation of bone turnover.