Hypokalemia (Low Potassium)

What potassium does

About 98% of the body's potassium is inside cells, with only a small amount in the blood. The difference in potassium across cell membranes creates the electrical charge that lets nerves fire, muscles contract and the heart beat in rhythm. The kidneys keep blood potassium in a narrow range, normally 3.5 to 5.0 mmol/L. Too little (hypokalemia) or too much (hyperkalemia) disturbs that electrical balance.

Symptoms of low potassium

Symptoms depend on how low the level is and how fast it fell:

Other effects:

• Irregular heartbeat, skipped beats or fast heart rate
• Tingling or numbness
• Bloating and slowed bowel (ileus)
• Passing large volumes of urine and thirst, when long-standing low potassium affects the kidneys
• Worse blood sugar control, because insulin release depends on potassium

Causes

Losses from the gut

• Vomiting, including repeated vomiting in eating disorders
• Diarrhea, including infections and inflammatory bowel disease
• Laxative misuse
• Drainage from tubes or fistulas

Vomiting lowers potassium partly through the kidneys: losing stomach acid causes alkalosis, which makes the kidneys waste potassium.

Losses through the kidneys

• Diuretics: thiazides (hydrochlorothiazide, chlorthalidone) and loop diuretics (furosemide). The most common cause in adults. See diuretic-induced electrolyte disorders.
• Low magnesium (hypomagnesemia): makes the kidneys lose potassium and makes it hard to correct until magnesium is replaced.
• Excess aldosterone: primary aldosteronism (Conn syndrome) causes high blood pressure with low potassium. Cushing syndrome and eating large amounts of real licorice act in a similar way.
• Inherited kidney tubule disorders: Gitelman syndrome and Bartter syndrome.
• Medicines: high-dose steroids, some antibiotics (amphotericin B, high-dose penicillins).
• Recovery from diabetic ketoacidosis treatment.

Shift of potassium into cells

• Insulin, especially during treatment of diabetic ketoacidosis
• Asthma inhalers and other beta-agonists, such as albuterol in high doses
• Caffeine or theophylline in large amounts
• Thyroid excess, causing thyrotoxic periodic paralysis
• Refeeding syndrome after starvation

Low intake

Low intake alone rarely causes hypokalemia, because the kidneys conserve potassium, but it adds to other causes in older adults, people with alcohol use disorder and those with eating disorders.

Diagnosis

1. Blood test: confirms the level. A falsely low result is uncommon, but a falsely normal or high result can occur if the sample is handled badly.
2. ECG: looks for flattened T waves, U waves and rhythm problems.
3. Magnesium, kidney function, glucose and bicarbonate.
4. Urine potassium: shows whether the kidneys are wasting potassium (pointing to diuretics, aldosterone excess or tubular disorders) or conserving it (pointing to gut losses).
5. Blood pressure: high blood pressure plus low potassium suggests excess aldosterone, and renin and aldosterone levels are checked.

Treatment

Replacing potassium

• Oral potassium chloride is preferred whenever possible. A doctor sets the dose and repeats blood tests. Tablets should be swallowed with water and food, sitting upright, as they can irritate the esophagus.
• IV potassium is used for severe levels, ECG changes, or when you cannot take tablets. It is given slowly with heart monitoring, because giving it too fast can stop the heart.
• Correct magnesium at the same time if low.

Do not take over-the-counter potassium supplements or salt substitutes (potassium chloride) without medical advice, especially if you have kidney disease or take ACE inhibitors, ARBs, spironolactone or similar drugs, which can cause dangerously high potassium.

Treating the cause

• Lower the diuretic dose, switch, or add a potassium-sparing diuretic such as spironolactone or amiloride.
• Treat vomiting, diarrhea or laxative use.
• Medicines or surgery for aldosterone-producing adrenal tumors.

Potassium-rich foods

Adults generally need about 2,600 to 3,400 mg of potassium a day from food. Good sources include:

• Beans, lentils and soybeans
• Potatoes and sweet potatoes with skin
• Bananas, oranges, cantaloupe, apricots and dried fruit
• Spinach, tomatoes, avocado and winter squash
• Milk, yogurt and fish

Diet alone usually cannot correct moderate or severe hypokalemia, but it helps prevent mild recurrence.

Complications

• Arrhythmias, including atrial and ventricular fibrillation
• Increased risk of digoxin toxicity
• Rhabdomyolysis and paralysis
• Kidney damage with long-standing low potassium
• Worsened glucose tolerance

Frequently asked questions

What are the symptoms of low potassium?

Mild low potassium often causes no symptoms. As levels fall you may notice tiredness, muscle weakness, cramps, constipation, palpitations or skipped heartbeats. Very low levels can cause paralysis and dangerous heart rhythms.

What is a dangerously low potassium level?

A level below 2.5 mmol/L is considered severe and potentially life-threatening, and needs urgent treatment, usually in hospital. Levels of 2.5 to 3.0 mmol/L also need prompt treatment, especially in people with heart disease or those taking digoxin.

What is the most common cause of low potassium?

In adults, diuretic medicines such as hydrochlorothiazide and furosemide are the most common cause. Vomiting and diarrhea are the next most common. Low magnesium often accompanies it and must also be corrected.

Can drinking too much water cause low potassium?

Drinking a lot of water mainly lowers sodium rather than potassium. However, heavy sweating, prolonged diarrhea or diuretics can lower potassium. Overhydration by itself is not a common cause of hypokalemia.

How quickly can low potassium be corrected?

Mild hypokalemia usually corrects within days of oral potassium. Severe hypokalemia is corrected over hours to days with IV and oral potassium under monitoring. If magnesium is also low, potassium will not stay up until magnesium is replaced.