Effect of vitamin D treatment in hypoparathyroid patients: a study on calcium, phosphate and magnesium homeostasis.

Abstract

This study was undertaken to examine the effects of long-term vitamin D treatment on calcium, phosphate and magnesium homeostasis at organ level in hypoparathyroid patients. Fifteen vitamin D-treated hypoparathyroid patients were studied, eight of the patients in a combined 47Ca kinetic and calcium, phosphate and magnesium balance study. Results were compared with a matched control group of 12 normal individuals. All the patients had normal serum levels of calcium, phosphate and magnesium. Absolute intestinal calcium absorption was increased (P < 0.0001). Urinary calcium excretion was normal, but active tubular calcium reabsorption (TmCa/glomerular filtration rate) was low (P< 0.001). Bone resorption rates and bone mineralization rates were very low (P < 0.001 and P < 0.05). Twenty-four-hour urinary hydroxyproline excretion and serum cross-linked carboxyterminal telopeptide of type I were in the upper normal range. Serum alkaline phosphatase was normal, but serum carboxyterminal propeptide of human type I procollagen and serum osteocalcin were significantly reduced (P < 0.05). Calcium balance was positive and significantly different from controls (P < 0.001). All parameters from phosphate homeostasis were normal. Intestinal magnesium absorption was low though not significantly different from normal (P = 0.06). Urinary excretion of magnesium was not significantly higher than normal, but renal magnesium reabsorption was reduced (P <0.001). Magnesium balance was low, though the difference was not significant (P < 0.06). Long-term vitamin D treatment in hypoparathyroid patients resulted in a positive calcium balance. Bone turnover was very low. Results of bone markers and resorption rate were conflicting. Vitamin D treatment apparently normalized the abnormalities previously found in phosphate homeostasis of hypoparathyroid patients. Magnesium homeostasis was disturbed, with a more negative balance compared with normal subjects, implying a state of magnesium deficiency in these patients.