[Phosphorus-calcium metabolism in hyperthyroidism]

Abstract

Because of the bone remodelling it induces, hyperthyroidism modifies the parameters of calcium-phosphorus metabolism. For a better determination of the mechanism involved, we studied 13 patients with Graves' disease compared with 13 controls. We measured the various parameters of calcium-phosphorus metabolism, notably the levels of parathormone, 25-hydroxycholecalciferol, 1-25 dihydroxycholecalciferol and ostocalcin; 8 patients were re-examined in euthyroidism. Total and corrected values of calcaemia (P less than 0.05 and P less than 0.01), phosphoreamie (P less than 0.01), alkaline phosphatase (P less than 0.01), calciuria (P less than 0.01) and hydroxyprolinuria (P less than 0.01) were significantly higher in patients with hyperthyroidism. Osteocalcin also was significantly increased (P less than 0.01) and correlated with thyroid hormone levels, thus confirming its usefulness as marker of bone remodelling in hyperthyroidism. Creatininaemia was significantly lowered (P less than 0.01). The intestinal absorption of calcium after injection of 1 g of calcium was reduced. Parathormone and 25-hydroxycholecalciferol levels were not significantly different in patients and in controls. In patients who were re-examined in euthyroidism, there was a significant increase in parathormone and in 1-25 dihydroxycholecalciferol levels (P less than 0.05). Thus, in situations of hyperthyroidism 2 elements contribute to a deficit in calcium balance: (a) a fall in parathormone level, consecutive to a rise in calcaemia, induces hypercalciuria; and (b) a fall in 1-25 dihydroxycholecalciferol level, consecutive to functional hypoparathyroidism and hyperphosphoraemia, results in a decrease of intestinal calcium absorption.