There is an important functional link between magnesium and calciotropic hormones. PTH stimulates magnesium reabsorption in the renal tubule, absorption in the gut and release of the ion from bone. Magnesium on the other hand is essential for the normal function of the parathyroid glands, metabolism of vitamin D and adequate sensitivity of target tissues to PTH and active vitamin D metabolites. Magnesium deficit is usually associated with hypoparathyroidism, low production of active vitamin D metabolites, in particular 1,25(OH)2 vitamin D3 and resistance to PTH and vitamin D. On the contrary, magnesium excess, similar to calcium, inhibits PTH secretion. Bone metabolism is impaired under positive as well as under negative magnesium balance. Because of the great importance of magnesium in the regulation of calcium homoeostasis, appropriate attention should be paid to the early diagnosis and treatment of magnesium imbalance.