Magnesium

Quantitative 24-hour urine magnesium (urinary magnesium excretion) is used to evaluate hypomagnesemia by separating renal magnesium wasting from extrarenal causes. It also assists in assessing tubular function, including salt-wasting tubulopathies, and in monitoring the effects of magnesium-wasting agents such as loop or thiazide diuretics, cisplatin, aminoglycosides, amphotericin, and cyclosporine. Interpretation is integrated with serum magnesium and renal function.

Magnesium is a predominantly intracellular cation and an essential cofactor for enzymes involved in ATP-dependent energy metabolism. Approximately 70% of total body magnesium resides in bone, most of the remainder is within cells, and only about 1% is present in blood. Renal transport in the thick ascending limb of the loop of Henle is central to magnesium balance, and urinary excretion reflects the kidney’s regulation of serum concentrations. Reduced glomerular filtration or excessive intake may lead to hypermagnesemia, whereas deficiency perturbs calcium, potassium, and phosphate homeostasis and can manifest with ventricular arrhythmias and neuromuscular or neuropsychiatric symptoms.