Biochemical effects of a calcium supplement in postmenopausal women with primary hyperparathyroidism.

Oral calcium loading is known to decrease parathyroid hormone levels in primary hyperparathyroidism. We have examined the effects of a calcium supplement on bone resorption in postmenopausal primary hyperparathyroidism. Fasting blood and urine samples were obtained in 12 postmenopausal women (median age 64 yr) with primary hyperparathyroidism associated with mild hypercalcemia (plasma calcium < 3.00 mmol/l). Further samples were obtained 12 hours after a 1 g calcium supplement given at 2100 h. After calcium administration there were rises in plasma ionized calcium (p < 0.02), plasma phosphate (p < 0.05) and the renal tubular maximum reabsorption capacity for phosphate (p < 0.01) and falls in parathyroid hormone (p < 0.05) and the renal tubular maximum reabsorption capacity for calcium (p < 0.05). The urinary calcium/creatinine increased (p < 0.01) and the urinary hydroxyproline/creatinine (p < 0.02) fell. These results indicate that calcium loading inhibits bone resorption in postmenopausal women with mild primary hyperparathyroidism.