Hypokalemia
Serum K <3.5 mEq/L; commonly from GI/renal losses; replace K and Mg.
Also known as: hypokalemia, low potassium
Overview
Serum potassium concentration <3.5 mEq/L. Severity: mild 3.0-3.4, moderate 2.5-2.9, severe <2.5. May reflect total body deficit (GI/renal loss, inadequate intake) or transcellular shift (alkalosis, insulin, beta-agonists).
Epidemiology
Most common electrolyte abnormality in clinical practice. Affects ~20% of hospitalized patients. Strong association with diuretic use, hyperaldosteronism, GI losses, and certain endocrine and renal tubular disorders.
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Risk factors
- Diuretics: thiazide (HCTZ, chlorthalidone, indapamide), loop (furosemide, torsemide, bumetanide)
- GI losses: vomiting, diarrhea, NG suction, laxative abuse
- Renal: hyperaldosteronism (primary, secondary), Cushing syndrome, Liddle syndrome, Bartter/Gitelman syndromes, RTA types 1 and 2
- Magnesium deficiency (renders K replacement ineffective)
- Transcellular shift: alkalosis (each 0.1 pH rise lowers K ~0.4), insulin, beta-2 agonists (albuterol), refeeding syndrome, hypokalemic periodic paralysis
- Decreased intake (rare alone): anorexia, alcoholism
- Drugs: amphotericin B, aminoglycosides, cisplatin, high-dose penicillins
Pathophysiology
Total body potassium deficit develops from loss exceeding intake (GI, renal) or from intracellular shift. Hypokalemia hyperpolarizes excitable membranes, prolongs repolarization (QT prolongation, U waves), and increases risk of re-entrant arrhythmia. In skeletal muscle, severe hypokalemia causes weakness and rhabdomyolysis. Renal tubular damage from chronic hypokalemia can cause nephrogenic DI.
Clinical presentation
Symptoms
- Mild: often asymptomatic
- Moderate: weakness, fatigue, muscle cramps, constipation
- Severe: flaccid paralysis (ascending), rhabdomyolysis, ileus, respiratory muscle weakness
- Cardiac: palpitations, syncope from arrhythmia
- Polyuria, polydipsia (nephrogenic DI from chronic hypokalemia)
Signs / physical exam
- Hyporeflexia in severe hypokalemia
- Hypotension (with concurrent volume depletion)
- Hypertension if hyperaldosteronism etiology
- Cardiac arrhythmias (premature beats, supraventricular and ventricular tachyarrhythmias, torsades de pointes if hypomagnesemia)
- Ileus
Classic findings
ECG: T-wave flattening or inversion, prominent U waves, ST depression, prolonged QT, possible AV block; predisposes to torsades especially with hypomagnesemia.
Differential diagnosis
- Diuretic-induced — Recent thiazide/loop initiation; urine K elevated; usually correctable with K replacement and oral supplementation
- GI loss (vomiting/diarrhea) — Recent GI illness; metabolic alkalosis (vomiting) or acidosis (diarrhea); urine K low (<20) if extrarenal
- Primary hyperaldosteronism — Hypertension + hypokalemia + metabolic alkalosis; aldosterone/renin ratio >20 with elevated aldosterone
- Bartter syndrome — Hereditary; hypokalemia + metabolic alkalosis + normal-low BP; loop-like; presents in childhood
- Gitelman syndrome — Hereditary; hypokalemia + alkalosis + hypomagnesemia + hypocalciuria; thiazide-like; often diagnosed in adolescence/adulthood
- RTA type 1 (distal) — Hypokalemia + non-gap metabolic acidosis + urine pH >5.5; kidney stones (calcium phosphate)
- RTA type 2 (proximal) — Hypokalemia + non-gap acidosis + glycosuria/aminoaciduria (Fanconi syndrome)
- Hypokalemic periodic paralysis — Episodic flaccid paralysis triggered by carbs/exercise; familial or thyrotoxic
Diagnostic workup
Diagnostic criteria
Serum K <3.5 mEq/L. Establish if renal vs extrarenal vs shift; assess concurrent acid-base, BP, and volume status to narrow differential.
Labs
- BMP — serum K, BMP, magnesium (essential — Mg deficit causes refractory hypokalemia)
- ABG or VBG for acid-base status
- Urine K (spot or 24-h) and urine creatinine — TTKG or urine K:Cr ratio distinguishes renal from extrarenal loss
- Urine K <20 mEq/L (spot) or <15 mEq/day (24-h) = extrarenal loss
- Urine K >20 mEq/L or >25 mEq/day = renal loss
- Plasma renin and aldosterone (aldo:renin ratio) if hypertension + hypokalemia
- TSH if periodic paralysis suspected
- Urine chloride (low in vomiting/contraction alkalosis; high in mineralocorticoid excess)
Imaging
- Adrenal CT/MRI if primary hyperaldosteronism confirmed by labs
Diagnostic algorithm
| Etiology Category | Urine K | Acid-Base | BP | Example |
|---|---|---|---|---|
| Extrarenal loss | Low (<20) | Acidosis (diarrhea) or normal | Variable | Diarrhea, laxative abuse |
| Vomiting / NG suction | Low-variable | Metabolic alkalosis | Often low | Bulimia, GI obstruction |
| Diuretic | High (>20) | Metabolic alkalosis | Variable | Thiazide, loop |
| Primary hyperaldosteronism | High | Metabolic alkalosis | HIGH | Conn adenoma |
| Bartter / Gitelman | High | Metabolic alkalosis | Normal/low | Inherited tubular defect |
| RTA type 1 or 2 | High | Non-gap metabolic ACIDOSIS | Normal | Sjögren (RTA-1), Fanconi (RTA-2) |
| Transcellular shift | Low (no loss) | Alkalosis or insulin effect | Variable | Insulin, beta-agonist, refeeding |
Treatment
First-line
- Replete potassium — oral if mild-moderate (K-Cl tablets 40-80 mEq/day in divided doses), IV if severe, symptomatic, NPO, or unable to tolerate oral
- IV KCl: typically max 10 mEq/h peripheral (40 mEq/L), max 20 mEq/h central; cardiac monitor for rates >10 mEq/h
- Each 10 mEq KCl raises serum K ~0.1 mEq/L (modest); large total body deficit (often >200-400 mEq) may exist with serum K of 3.0
- REPLACE MAGNESIUM concurrently — hypomagnesemia causes K wasting and renders K supplementation ineffective. Goal Mg >2.0 mg/dL.
- Address underlying cause: stop laxative, switch diuretic, treat hyperaldosteronism
- K-sparing diuretic — spironolactone, eplerenone, amiloride, triamterene — when ongoing renal losses
- Dietary K — bananas, oranges, potatoes, leafy greens, salt substitutes
Second-line / adjunct
- Primary hyperaldosteronism: spironolactone or eplerenone; adrenalectomy for unilateral adenoma
- Bartter/Gitelman: K and Mg replacement; spironolactone or amiloride; NSAIDs (Bartter only)
- RTA: bicarbonate replacement with K supplementation
- Thyrotoxic periodic paralysis: treat hyperthyroidism, beta-blocker (non-selective), K cautiously
- Refeeding syndrome: gradual nutritional advancement, prophylactic phosphorus, K, Mg, thiamine
- Recheck K every 2-6 hours during active replacement
Complications
- Cardiac arrhythmias: PVCs, atrial and ventricular tachyarrhythmias, torsades de pointes
- Sudden cardiac death (especially with concurrent QT-prolonging medications)
- Muscle weakness, rhabdomyolysis
- Respiratory failure (severe)
- Ileus, constipation
- Nephrogenic diabetes insipidus, polyuria
- Hypertension if etiology is mineralocorticoid excess
- Digoxin toxicity (hypokalemia potentiates) — important in patients on digoxin
PANCE pearls
- ALWAYS check and replace magnesium when treating hypokalemia. Hypomagnesemia causes renal K wasting and makes K supplementation ineffective.
- Rule of thumb: each 10 mEq KCl raises serum K only ~0.1 mEq/L. Patients with serum K 3.0 often have a total body deficit of 200-400 mEq.
- Hypokalemia potentiates digoxin toxicity even at therapeutic levels — maintain K >4 in patients on digoxin.
- ECG: T-wave flattening, U waves, ST depression, QT prolongation, predisposing to torsades.
- Hypertension + hypokalemia + metabolic alkalosis = think primary hyperaldosteronism (Conn syndrome). Check ARR.
- Gitelman syndrome: hypokalemia + alkalosis + low Mg + low urinary calcium. Bartter syndrome: similar but with normal/high urinary calcium and presents earlier.
References
- KDIGO 2020 — KDIGO Controversies Conference on Potassium Management in Kidney Disease
- Mount Krapf — Disorders of Potassium Balance (Mount, in Brenner & Rector's The Kidney, 11th ed)
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