Magnesium is the second most abundant intracellular cation after potassium and is a cofactor for over 300 enzymatic reactions, including ATP synthesis, DNA/RNA synthesis, protein synthesis, and virtually all reactions involving ATP. Serum magnesium represents only approximately 1% of total body magnesium — the vast majority is intracellular or in bone. This means that serum magnesium can be normal even when significant intracellular deficiency exists, and conversely, serum magnesium can be low in the context of a severe total body deficit. The clinical implication: a patient with a serum Mg of 0.65 mmol/L (low-normal) but with persistent symptoms (muscle cramps, fatigue, palpitations) and risk factors for depletion (PPI use, alcohol, malabsorption) may have significant total body Mg deficiency that is not captured by the serum level. The refractory hypokalaemia principle is one of the most clinically important relationships in electrolyte physiology: magnesium is required as a cofactor for the Na/K-ATPase pump that maintains intracellular potassium. Without adequate magnesium, potassium leaks out of cells into the urine through the ROMK channel in the distal tubule. Any patient with persistent hypokalaemia despite adequate potassium supplementation must have their serum magnesium checked — treating the hypomagnesaemia is the prerequisite for correcting the hypokalaemia.

PPI-induced hypomagnesaemia is one of the most common and most under-recognised causes of hypomagnesaemia in primary care — affecting approximately 1–4% of long-term PPI users. The mechanism involves impaired active magnesium absorption via the TRPM6 transporter in the small intestinal epithelium, which requires an acidic environment to function optimally. PPIs suppress gastric acid, reducing intestinal TRPM6 activity and passive Mg diffusion. The risk increases with: dose (omeprazole 40 mg > 20 mg), duration (risk appears after approximately 1 year), concomitant diuretics (additive renal loss), and older age. MHRA issued a guidance in 2012 recommending that serum Mg should be checked before starting PPI therapy and periodically during long-term use (annually or when symptoms occur). GPs prescribing long-term PPIs should check annual serum magnesium as part of medication monitoring. If hypomagnesaemia is confirmed on PPI therapy: consider switching to H2 blocker (ranitidine/famotidine — do not impair Mg absorption), reduce PPI dose if possible, or continue PPI with oral Mg supplementation if switching is not clinically appropriate. The hypomagnesaemia resolves within weeks of PPI discontinuation in most cases.

The 24-hour urine magnesium test is the most useful investigation for distinguishing renal from extra-renal causes of hypomagnesaemia — it directly measures how much magnesium the kidney is excreting. In hypomagnesaemia due to GI loss or inadequate intake, the kidney appropriately conserves magnesium (urine Mg <1 mmol/24h = appropriate renal conservation). In hypomagnesaemia due to renal wasting (loop diuretics, cisplatin, aminoglycosides, Gitelman syndrome, primary aldosteronism), the kidney inappropriately excretes magnesium despite low serum levels (urine Mg >1 mmol/24h = renal wasting). This distinction guides management: renal wasting requires addressing the cause (changing diuretic, stopping nephrotoxic drug, treating aldosteronism) in addition to replacement; GI loss requires treating the GI condition + supplementation. The fractional excretion of Mg (FEMg) is a spot urine alternative that does not require a 24-hour collection and provides equivalent information.

The torsades de pointes-magnesium connection is one of the most important pharmacological relationships in acute medicine — torsades de pointes (TdP) is a potentially lethal polymorphic ventricular tachycardia associated with QT prolongation (the twisted points pattern on ECG where QRS complexes appear to twist around the isoelectric line). The first-line treatment for TdP regardless of the serum magnesium level is IV MgSO₄ 2g (8 mmol) over 1–2 minutes, followed by an infusion. The mechanism: magnesium blocks the cardiac L-type calcium channel and stabilises the cardiac action potential. Magnesium terminates TdP in approximately 80% of cases. GPs must know this for two reasons: (1) any patient on QT-prolonging drugs (antipsychotics, antifungals, antimalarials, methadone, many antiarrhythmics) who is also at risk of hypomagnesaemia (PPI, diuretics, alcohol, diarrhoea) has compounded QT prolongation risk — serum Mg should be checked and maintained in the normal range; (2) GPs must not prescribe QT-prolonging drugs to patients with known hypomagnesaemia without correcting the Mg first.

Gitelman syndrome is an important differential diagnosis in young patients with recurrent or unexplained hypomagnesaemia — it is an autosomal recessive tubular disorder (SLC12A3 gene — thiazide-sensitive Na/Cl cotransporter) causing: hypomagnesaemia (always present), hypokalaemia, hypocalciuria (low urine calcium — distinguishes from Bartter syndrome), and metabolic alkalosis. It is often discovered incidentally on routine bloods in young adults, but can present with: muscle weakness, cramps, tetany, fatigue, and palpitations. The diagnosis should be suspected in any patient under 40 with unexplained hypomagnesaemia + hypokalaemia + metabolic alkalosis, particularly if the urine calcium is low. The treatment: lifelong oral magnesium supplementation (Mg glycerophosphate — better tolerated than Mg oxide) + dietary salt and potassium supplementation. Amiloride (potassium-sparing diuretic that also reduces renal Mg wasting) is a useful adjunct. Nephrology referral for genetic confirmation and management optimisation.

Magnesium glycerophosphate is the preferred oral magnesium preparation in UK clinical practice for two reasons: (1) it has significantly better GI tolerability than magnesium oxide or magnesium sulphate (both of which cause dose-dependent osmotic diarrhoea — which can worsen hypomagnesaemia if the cause is GI); (2) it provides both magnesium and glycerophosphate, supporting ATP synthesis. The licensed product (Neuro-Phosph, or 97.2 mg tablets) is available on FP10 prescription. Magnesium oxide (400 mg tablets, available OTC) is an acceptable alternative for mild deficiency where cost is a concern, but patients should be warned that it may cause loose stools, particularly at higher doses. The dosing principle: start at the lower end of the dose range and titrate up based on serum Mg response and GI tolerability. Doses above 15–20 mmol/day orally rarely achieve further improvement because the gut reaches maximum absorptive capacity — higher doses simply cause diarrhoea. IV supplementation is required for moderate-severe deficiency or when GI absorption is compromised.

The alcohol withdrawal protocol and magnesium connection is important for GPs managing inpatient or outpatient alcohol detoxification — standard chlordiazepoxide CIWA protocols do not include magnesium supplementation, but Mg deficiency is nearly universal in chronic alcoholics and contributes significantly to withdrawal symptoms (tremor, anxiety, seizures, cardiac arrhythmias). IV Pabrinex (which contains thiamine, riboflavin, pyridoxine, nicotinamide, and ascorbic acid) does not contain magnesium. Separate IV magnesium supplementation should be added to the withdrawal protocol for any patient with confirmed hypomagnesaemia or those at high risk (heavy alcohol dependence, poor nutrition, recent vomiting/diarrhoea). IV MgSO₄ 2g (8 mmol) twice daily during the first 3–5 days of alcohol detoxification is a reasonable empirical approach when serum Mg cannot be checked immediately. This reduces the risk of withdrawal seizures, which are partly driven by hypomagnesaemia-related NMDA receptor hyperactivity.

The dietary magnesium insufficiency across the UK population is a genuine public health concern — NDNS (National Diet and Nutrition Survey) data consistently shows that a significant proportion of UK adults have magnesium intakes below the lower reference nutrient intake (LRNI), with older adults, adolescent girls, and people with low socioeconomic status particularly affected. The consequences manifest as: increased risk of hypertension (Mg is a physiological vasodilator), insulin resistance and T2DM, depression (Mg modulates NMDA receptors and serotonergic neurotransmission), migraine (Mg supplementation at 400–600 mg/day reduces migraine frequency by approximately 40% in deficient patients — EFNS guidelines recommend Mg as a preventive option), and osteoporosis. GPs advising patients on lifestyle modifications for any of these conditions should routinely include dietary magnesium optimisation as part of the guidance.

The interaction between hypomagnesaemia and digoxin toxicity is a crucial prescribing safety point — magnesium deficiency increases myocardial sensitivity to digoxin toxicity. Hypomagnesaemia reduces Na/K-ATPase activity in cardiomyocytes, which is the same enzyme inhibited by digoxin. The combined inhibition from Mg deficiency + digoxin leads to: increased intracellular calcium (the toxic mechanism of digoxin toxicity) at lower digoxin concentrations than usual. This means a patient with hypomagnesaemia can develop digoxin toxicity at serum digoxin levels considered therapeutic in a normomagnesaemic patient. GPs prescribing digoxin to patients who are also on loop diuretics or PPIs (Mg-depleting) must monitor serum Mg regularly and maintain it in the normal range. Any patient on digoxin presenting with nausea, visual disturbances, bradyarrhythmia, or altered cognition should have serum digoxin level AND serum Mg measured simultaneously.