Hypomagnesemia

A patient's potassium keeps dropping despite aggressive replacement — the hidden culprit might be a magnesium level no one checked. You can't fix one without the other.

Magnesium (normal 1.5–2.5 mEq/L) is a gatekeeper electrolyte: it stabilizes cardiac cell membranes, facilitates potassium uptake into cells, and is required for PTH secretion and calcium regulation. When magnesium falls below 1.5 mEq/L, three downstream problems emerge. First, refractory hypokalemia — the kidneys waste potassium through ROMK channels that magnesium normally blocks, so potassium replacement fails until magnesium is corrected. Second, hypocalcemia — low magnesium suppresses PTH release and causes end-organ resistance to PTH, dropping calcium simultaneously. Third, neuromuscular hyperexcitability — tremors, hyperactive deep tendon reflexes, positive Trousseau's and Chvostek's signs (shared with hypocalcemia because magnesium deficiency causes it). Cardiac effects include prolonged QT interval, torsades de pointes, and increased digoxin sensitivity. The most common causes are chronic alcohol use, loop and thiazide diuretics, proton pump inhibitors with prolonged use, and GI losses (diarrhea, NG suction). IV magnesium sulfate is the replacement route for severe deficiency (< 1.0 mEq/L) or symptomatic clients — administered slowly with continuous cardiac monitoring because rapid infusion causes hypotension and respiratory depression. Oral magnesium causes diarrhea, which paradoxically worsens losses.

Key Distinctions

Don't confuse hypomagnesemia's hyperexcitable presentation (tremors, seizures, hyperreflexia) with hypermagnesemia's sedated presentation (hyporeflexia, respiratory depression) — they are mirror opposites. Students often treat refractory hypokalemia by giving more potassium; the correct move is checking and replacing magnesium first. Trousseau's and Chvostek's signs appear in both hypomagnesemia and hypocalcemia — low magnesium is frequently the upstream cause of the low calcium.

Clinical Pearl

Mag is the mother electrolyte: if potassium won't come up or calcium won't come up, check magnesium first — it controls them both.