Magnesium metabolism disturbances Magnesium (Mg) is the second most abundant intracellular cation. Only 1% of Mg is in the extracellular fluid and the plasma Mg concentration does not reflect a substantial tissue depletion. Hypomagnesemia is the most common abnormality of Mg balance. This disorder is often associated with hypocalcemia, hypokalemia and metabolic alkalosis. Hypomagnesemia must be suspected in any patient with alcoholism, chronic diarrhoea or on diuretic. The 24-hour urinary Mg excretion of greater than 10 to 30 mg (1 mmol) or a calculated fractional excretion of Mg of greater than 2% suggest inappropriate renal wasting. Symptomatic hypomagnesemia must be treated by intravenous Mg, following by oral intake of Mg salt and by Mg rich diet. Hypermagnesemia is rare and occurs in case of high load and/or when renal function is impaired.