Extracellularly, magnesium ions block neurosynaptic transmission by interfering with the release of acetylcholine. Magnesium ions also may interfere with the release of catecholamines from the adrenal medulla. Magnesium has been proposed as an endogenous endocrine modulator of the catecholamine component of the physiologic stress response. No accurate method exists to assess ionized serum magnesium. Infants and children tend to have higher daily requirements.
Elimination is predominantly renal.
The threshold for urinary excretion is near the normal serum concentration. Conversely, the kidney retains a strong capacity to resorb magnesium, and the main site for reabsorption is the thick ascending loop of Henle. Of these 3 types of diuretics, loop diuretics have the greatest effect on renal magnesium wasting because of their site of action. Prolonged administration of magnesium-free parenteral fluids may be a contributing factor. Prolonged nasogastric suction, infectious diarrhea, steatorrhea, inflammatory bowel disease, and GI neoplasms may cause hypomagnesemia.