Magnesium, calcium and PTH relationships in dialysis patients after magnesium repletion

Sixteen end-stage renal disease patients who received hemodialysis for 2.9 years with no magnesium in the dialysate were evaluated for serum calcium, magnesium and parathyroid hormone (PTH) status. Eleven of these patients became hypomagnesemic with subnormal total and ultrafiltrable serum Mg concentrations (low-Mg group). The five remaining patients had consistently normal serum Mg concentrations (normal-Mg group). All had normal total and ionized serum Ca. However, the total and ionized serum Ca levels in the low-Mg group were significantly lower than those of the normal-Mg group. Within 2 weeks of Mg repletion (by dialysis against 0.25 mM of Mg), the N-terminal PTH, total and ultrafiltrable serum Mg, and the total and ionized serum Ca increased in the low-Mg group. There was a strong negative correlation between initial serum Mg and the percent change in N-terminal PTH (r = 0.833) at 2 weeks for all patients. There was also a strong positive correlation between initial ionized Ca and total serum Mg for all patients (r = 0.774). Mg repletion decreased the incidence of cramps experienced during dialysis by all patients. These data suggest that Mg-free dialysis in some patients can cause Mg depletion, which is symptomatic and which may suppress the parathyroid gland.