Electrolytes: sodium, potassium, calcium, magnesium

This is a standalone electrolytes chapter, not a single-question rationale. Learn each ion the same way: what it does in the body, what low looks like and why it happens, how you treat low, then the same for high. Once that map is solid, NCLEX items become pattern recognition.

The four ions that carry most of the exam

Volume status (dry vs wet) and electrolytes travel together, but they are not the same skill. Volume asks how much water is in the tank. Electrolytes ask which charged particle is putting the brain, heart, airway, or reflexes at risk right now.

• Sodium: osmolarity and brain water.
• Potassium: resting membrane potential and cardiac rhythm.
• Calcium: neuromuscular excitability and clotting support.
• Magnesium: calms neuromuscular transmission; overdose flattens reflexes and breathing.

Memory cards (scan these first)

Four high-contrast cards for fast recall. Cover one column and say the other out loud, then read the full sections below.

Sodium (Na+)

Role in the body

Sodium is the main extracellular cation. It holds water outside cells and sets serum osmolarity. When sodium falls relative to water, water shifts into brain cells. When sodium rises relative to water, cells shrink. That is why sodium questions are almost always neurologic first.

Typical adult serum sodium sits near 135–145 mEq/L. Treat the clinical picture and the rate of change, not a single digit in isolation.

Hyponatremia (low sodium)

What it means: too little sodium relative to body water, so brain cells swell.

Common causes you will see tested:

• SIADH (too much ADH → water retention that dilutes sodium)
• Excess hypotonic IV fluids or free-water intake
• Thiazide diuretics
• Heart failure, cirrhosis, or kidney disease with diluted serum sodium
• GI losses with hypotonic replacement

Signs / priorities: headache, confusion, lethargy, nausea, seizures, and in severe cases coma. Fall and seizure precautions matter when the neuro exam is changing.

Treatment logic:

• Mild, asymptomatic, euvolemic SIADH pattern: fluid restriction is often first-line.
• Hypovolemic hyponatremia: restore volume with isotonic saline as ordered.
• Severe symptomatic hyponatremia (seizures, deep neuro change): controlled hypertonic (3%) saline in a monitored setting, with frequent sodium and neuro checks.
• Correct slowly. Fast jumps risk osmotic demyelination. Do not delay emergency treatment for a perfect cause workup when the client is seizing, but do not race the sodium upward once the emergency is controlled.

Food / diet angle:

• Sodium is abundant in table salt, canned soups, processed meats, chips, soy sauce, and restaurant meals. Hypovolemic / salt-loss hyponatremia may allow normal dietary salt once volume is restored.
• In SIADH and fluid-overload hyponatremia, the teaching priority is usually fluid restriction, not loading salt at the bedside. Do not coach unlimited salty snacks when free water is the problem.
• NCLEX diet items for sodium more often test restriction (heart failure, hypertension, SIADH) than “eat more salt.”

Hypernatremia (high sodium)

What it means: too much sodium relative to water, usually a water deficit (or a large salt load).

Common causes:

• Inadequate free-water intake (older adults with blunted thirst)
• Insensible losses: fever, tachypnea, burns
• Diabetes insipidus (large dilute urine water loss)
• Osmotic diarrhea or osmotic diuresis
• Hypertonic saline or sodium bicarbonate loads

Signs / priorities: intense thirst (if the thirst mechanism works), dry mucous membranes, restlessness or irritability, and progressive neuro change as cells shrink.

Treatment logic:

• Restore free water gradually with oral water or hypotonic IV fluids as ordered.
• Treat the driver (for example, desmopressin pathways in central DI when that is the diagnosis).
• Question giving more hypertonic saline when the problem is already a water deficit / high sodium.
• Correct gradually to avoid cerebral edema from an overly fast drop in sodium.

Food / diet angle:

• Cut back on high-sodium processed foods and salt packets while free-water replacement is underway.
• Encourage scheduled oral water (unless restricted for another reason). Older adults may need prompting because thirst declines with age.

Potassium (K+)

Role in the body

Potassium is the main intracellular cation. The small amount in serum controls the resting membrane potential of nerve and muscle, especially cardiac muscle. That is why potassium questions are cardiac and muscle first.

Typical adult serum potassium sits near 3.5–5.0 mEq/L. Small shifts matter. Potassium is never given as an IV push.

Hypokalemia (low potassium)

Common causes:

• Loop and thiazide diuretics
• GI losses: vomiting, NG suction, diarrhea
• Shift into cells: insulin, alkalosis, beta-agonists
• Poor intake, magnesium depletion (K will not hold until Mg is fixed)

Signs / priorities: muscle weakness, cramps, fatigue, constipation/ileus cues, and ECG changes such as flattened T waves and U waves. Digoxin toxicity risk rises when potassium is low.

Treatment logic:

• Cardiac monitoring when low or symptomatic.
• Replace potassium orally when possible; use controlled IV infusion when needed.
• Never IV push potassium.
• Check and replace magnesium if low.
• Hold or adjust digoxin per protocol when toxicity risk is high; treat the potassium first when that is the driver.

Potassium-rich foods (teach for hypoK / diuretic teaching):

• Fruits: banana, orange / orange juice, cantaloupe, apricot, dried fruit (raisins, prunes).
• Vegetables: potato (especially with skin), tomato / tomato products, spinach, avocado, sweet potato.
• Other: beans and lentils, yogurt, salt substitutes that contain potassium chloride (only if ordered / not contraindicated).

Hyperkalemia (high potassium)

Common causes:

• Kidney failure / decreased excretion
• ACE inhibitors, ARBs, potassium-sparing diuretics
• Shift out of cells: acidosis, massive cell lysis, succinylcholine in at-risk clients
• Excess replacement

Signs / priorities: muscle weakness can appear, but the exam killer finding is the ECG: peaked T waves, prolonged PR, widened QRS, and risk of sine-wave / arrest progression.

Treatment sequence (memorize the order):

1. Protect the heart if ECG changes are present: IV calcium (gluconate or chloride per protocol) stabilizes membranes.
2. Shift potassium into cells: insulin with glucose, beta-agonists, and/or bicarbonate in selected acidosis cases.
3. Remove potassium from the body: potassium-binding resins, diuretics if the kidneys work, or dialysis when needed.
4. Stop ongoing potassium intake and review drugs that raise potassium.

Food / diet angle (hyperK / CKD):

• Start a low-potassium diet: limit bananas, oranges, potatoes, tomatoes, spinach, avocado, dried fruit, and beans as ordered.
• Avoid salt substitutes made with potassium chloride unless the provider clears them.
• Acute ECG-changing hyperkalemia is treated with meds/dialysis first; diet teaching supports prevention and chronic management.

Calcium (Ca2+)

Role in the body

Calcium stabilizes nerve membranes, enables muscle contraction coupling, supports clotting, and builds bone. Ionized calcium is the physiologically active fraction. When calcium falls, nerves fire too easily. When calcium rises, nerves and muscles quiet down.

Typical total serum calcium sits near 8.5–10.5 mg/dL, but always interpret with albumin and the clinical picture. Low albumin can lower total calcium without the same ionized effect.

Hypocalcemia (low calcium)

Common causes:

• Hypoparathyroidism or accidental parathyroid injury after thyroid/neck surgery
• Chronic kidney disease (impaired activation of vitamin D)
• Vitamin D deficiency
• Acute pancreatitis
• Massive transfusion (citrate binding)
• Severe hypomagnesemia

Signs / priorities: perioral numbness, finger tingling, muscle cramps, twitching, hyperactive reflexes, Chvostek and Trousseau signs, and the airway emergency of laryngospasm. Prolonged QT can appear on ECG.

Treatment logic:

• Airway readiness and seizure/tetany precautions.
• Replace calcium (oral for mild; IV calcium for symptomatic / severe).
• Treat the driver (Mg, vitamin D, CKD pathway) so calcium stays up.
• After neck surgery, new tingling or stridor is a now problem, not a wait-and-see lab.

Calcium-rich foods (teach for mild hypoCa / long-term intake):

• Dairy: milk, yogurt, cheese.
• Alternatives: calcium-fortified plant milks and juices, tofu set with calcium, canned salmon or sardines with bones.
• Greens: kale and collards (absorption varies; spinach is calcium-rich but oxalate can limit absorption).
• Pair teaching with vitamin D (sun, fortified milk, fatty fish) because calcium absorption depends on it.

Hypercalcemia (high calcium)

Common causes:

• Hyperparathyroidism
• Malignancy-related calcium release
• Vitamin D excess / granulomatous disease
• Thiazides, lithium, prolonged immobilization

Signs / priorities: “stones, bones, groans, psychiatric overtones” in the classic sense — kidney stones, bone pain, constipation/abdominal discomfort, and lethargy or confusion. Muscles are weak. Polyuria and dehydration can follow.

Treatment logic:

• Aggressive IV hydration as ordered (usually isotonic saline) to restore volume and promote calcium excretion.
• Stop contributors (thiazides, excess calcium/vitamin D) when appropriate.
• Medications such as bisphosphonates or calcitonin appear in selected severe / malignancy pathways.
• Fall precautions while weakness and lethargy are present; treat the underlying driver.

Food / diet angle:

• Hold extra calcium and vitamin D supplements until cleared.
• Encourage oral fluids as ordered; diet alone rarely fixes severe hypercalcemia from malignancy or hyperparathyroidism.

Magnesium (Mg2+)

Role in the body

Magnesium stabilizes neuromuscular transmission and supports countless enzyme systems, including those that move potassium into cells. Think of magnesium as a calming cofactor. Too little, and the system becomes irritable. Too much, and reflexes and breathing flatten.

Typical adult serum magnesium sits near 1.5–2.5 mg/dL (lab ranges vary slightly). Obstetric magnesium infusions make toxicity a favorite exam scenario.

Hypomagnesemia (low magnesium)

Common causes:

• Alcohol use disorder and malnutrition
• GI losses: diarrhea, steatorrhea, fistulas
• Loop/thiazide diuretics
• Refeeding and some chemotherapy exposures

Signs / priorities: tremors, tetany-like irritability, hyperactive reflexes, seizures when severe, and dysrhythmias. Hypokalemia and hypocalcemia often travel with it.

Treatment logic:

• Replace magnesium (oral or IV depending on severity).
• Monitor cardiac rhythm when low or being replaced IV.
• If potassium will not correct, check magnesium.
• Seizure precautions when neuromuscular irritability is high.

Magnesium-rich foods (teach for hypoMg / poor intake):

• Nuts and seeds: almonds, cashews, pumpkin seeds.
• Grains and legumes: whole grains, black beans, lentils.
• Greens: spinach and other leafy greens.
• Other: avocado, dark chocolate (in moderation), fortified cereals.

Hypermagnesemia / magnesium toxicity (high magnesium)

Common causes:

• Magnesium infusions (preeclampsia / eclampsia protocols)
• Excess magnesium-containing antacids or laxatives in kidney failure
• Impaired renal excretion

Signs / priorities: warmth and flushing early, then loss of deep tendon reflexes, progressive muscle weakness, hypotension, and respiratory depression. This is a stop-the-infusion emergency pattern.

Treatment sequence:

1. Stop the magnesium infusion / source immediately.
2. Support airway and breathing; call for help per protocol.
3. Give calcium gluconate as the antidote to antagonize magnesium at the neuromuscular junction / cardiac membrane.
4. Promote excretion (fluids/diuretics if kidneys work) or dialysis when needed.

Food / diet angle (hyperMg):

• In kidney failure, teach clients to avoid magnesium-containing antacids and laxatives (common hidden source).
• Hold magnesium-rich supplements when levels are high; acute toxicity on an infusion is managed by stopping the drip, not by diet tweaks.

Electrolyte-rich foods in treatment

Diet teaching is part of treatment, not a side note. For low levels, name foods that rebuild the ion when the gut works and the situation is not an emergency. For high levels, name what to restrict. Emergencies still get meds and monitoring first. Scan the four food cards above in each ion section, then use this table to self-test.

Cross-cutting rules that win items

Pairs that travel together

• Low magnesium often keeps potassium (and sometimes calcium) low until Mg is replaced.
• Acidosis tends to raise serum potassium; alkalosis tends to lower it (shifts).
• Volume resuscitation with lots of insulin/glucose or bicarbonate can drop potassium — watch the trend.
• After thyroid/parathyroid surgery, new tingling is hypocalcemia until proven otherwise.

How to reason under time pressure

1. Name the ion.
2. Name the organ at risk (brain, heart, airway, reflexes).
3. Decide low vs high from the stem cues.
4. Pick the first protective action, then the definitive fix.

Quick revision sheet

Print this page and cover the treatment column. Say the role, then hypo, then hyper, out loud before you check yourself.

Related shorter notes and question practice stay attached through topic tags for potassium/sodium, calcium/magnesium, and fluid–electrolyte items. Use this chapter when you want the full map, not a one-item explanation.