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.
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.