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Hypomagnesaemia is defined in terms of a lower than normal free serum ionised magnesium concentration (0.54–0.67 mmol/l), although may be confused with total body magnesium deficiency which is possible with normal serum magnesium, and can occur with normal total body magnesium. It is grossly underdiagnosed, being present in more than 10% of acute inpatients, and this increases with the severity of the illness[1]. There is a lack of correlation in critical ill patients between albumin, serum total magnesium as usually measured and free ionised magnesium[2].

Info bulb.pngLocal protocols will vary, but an easy way to remember how to replace magnesium is that everything has 5 in it. So 5g magnesium sulfate (magnesium sulphate) in 500ml of 5% glucose over 5 hours (=4mmol/hour replacement).



  • Endocrine disease
Info bulb.pngPPIs may be followed by hypomagnesaemia.[3] The mechanism is unclear although there is evidence that PPIs can inhibit active magnesium transport in the intestine[4].


Signs and symptoms unusual if serum magnesium > 0.5 mmol/l

LogoKeyPointsBox.pngTo correct hypokalaemia secondary to hypomagnesaemia you must correct the hypomagnesaemia.


  • Treat underlying condition
  • Acute symptomatic hypomagnesaemia must be treated with iv magnesium salt (monitoring may be necessary especially in renal failure)
  • Chronic magnesium deficiency states may need oral magnesium gluconate as well as appropriate diet