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Normal plasma magnesium concentration is 1.8–3.0 mg/dL (or 0.75–1.25 mmol/L), with about one-third bound to protein and two-thirds existing as free cation. Magnesium excretion is via the kidney. Magnesium’s physiologic effects on the nervous system resemble those of calcium.
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Altered magnesium concentration usually provokes an associated alteration of Ca2+. Both hypomagnesemia and hypermagnesemia can decrease PTH secretion or action. Severe hypermagnesemia (greater than 5 mg/dL [2.1 mmol/L]) suppresses PTH secretion with consequent hypocalcemia; this disorder is typically seen only in patients receiving magnesium therapy for preeclampsia. Severe hypomagnesemia causes PTH resistance in end organs and eventually decreased PTH secretion in severe cases.