Hyponatremia and Hypernatremia in Kidney Disease: What You Need to Know

When your kidneys start to fail, even small changes in your body’s sodium levels can become dangerous. Hyponatremia (low sodium) and hypernatremia (high sodium) aren’t just lab numbers-they’re real risks that can lead to falls, confusion, hospital stays, and even death in people with chronic kidney disease (CKD). Around 1 in 5 people with advanced CKD will develop one of these sodium disorders, and many don’t even know it until something serious happens.

Why Your Kidneys Control Your Sodium

Your kidneys don’t just filter waste-they’re your body’s main sodium and water regulators. Healthy kidneys adjust urine concentration based on how much salt and water you take in. If you drink too much water, they make dilute urine to get rid of the excess. If you’re low on water, they save it by making concentrated urine. This balance keeps your blood sodium between 135 and 145 mmol/L.

But when CKD progresses, this system breaks down. By stage 4 or 5 (GFR under 30 mL/min), your kidneys can’t make concentrated urine anymore. They also struggle to flush out extra water. That means even a small increase in fluid intake can flood your system with water, diluting your sodium and causing hyponatremia. On the flip side, if you’re not drinking enough-or you’re losing water through fever, sweating, or medications-your body can’t replace it properly, and sodium climbs too high, leading to hypernatremia.

Hyponatremia: The Silent Threat in CKD

Hyponatremia-serum sodium below 135 mmol/L-is the most common sodium disorder in CKD, affecting 60-65% of cases. The biggest culprit? Your kidneys can’t excrete water fast enough. This gets worse if you’re on thiazide diuretics (like hydrochlorothiazide), which are often prescribed for high blood pressure but become ineffective and risky once GFR drops below 30.

Many patients with CKD are told to cut back on salt, protein, and potassium to protect their kidneys. But cutting back too much can backfire. A 2023 Japanese study found that strict solute restriction lowers urine output, making it even harder for kidneys to get rid of excess water. This pushes sodium levels down further. In fact, 22% of hyponatremia cases in late-stage CKD patients are linked to overly aggressive dietary restrictions.

The symptoms are subtle at first: fatigue, nausea, headaches. But over time, low sodium causes brain swelling. Studies show hyponatremic CKD patients have a 35% higher risk of falls, a 67% higher risk of fractures, and nearly double the risk of death compared to those with normal sodium levels. Hospitalized patients with hyponatremia have a 28% higher mortality rate than those without it.

Hypernatremia: When You’re Too Dry

Hypernatremia-sodium above 145 mmol/L-is less common but just as dangerous. It usually happens when you’re not drinking enough water, or when you’re losing too much fluid through vomiting, diarrhea, or fever. In CKD, your kidneys can’t concentrate urine to save water, so you lose more than you should. Elderly patients with dementia or mobility issues are especially at risk because they may not feel thirsty or can’t reach water on their own.

High sodium pulls water out of your brain cells, causing them to shrink. This can lead to confusion, seizures, coma, and even permanent brain damage. The key is to correct it slowly-no faster than 10 mmol/L per day. Rushing correction can cause cerebral edema, which is just as deadly as the original high sodium.

Three Types of Hyponatremia in CKD

Not all hyponatremia is the same. In CKD, it breaks down into three types:

• Hypovolemic (15-20%): You’ve lost both sodium and water, but more sodium. This can happen with overuse of diuretics, vomiting, or rare salt-wasting conditions like milk-alkali syndrome.
• Euvolemic (60-65%): Your total body water is high, but your sodium level is low. This is the most common type in CKD. Your kidneys can’t excrete water, so fluid builds up. Thiazide diuretics are a major cause here.
• Hypervolemic (15-20%): You have too much total fluid and too much sodium, but the water is overwhelming the sodium. This happens in advanced CKD with severe swelling (edema) or when heart failure is also present.

Knowing which type you have changes how you treat it. Giving fluids to someone with hypervolemic hyponatremia makes it worse. Restricting fluids in someone with hypovolemic hyponatremia can be deadly.

Treatment: Less Is Often More

There’s no one-size-fits-all fix. Treatment has to match your kidney function, your symptoms, and your volume status.

For hyponatremia: Fluid restriction is the first step. If you’re in stage 4 or 5 CKD, limit fluids to 800-1,000 mL per day. In earlier stages, you might be allowed up to 1,500 mL. Never drink more than your kidneys can handle. Sodium supplements are rarely needed-unless you have a salt-wasting condition, in which case 4-8 grams of sodium chloride per day may be prescribed. Correction speed is critical: raise sodium no more than 4-6 mmol/L in 24 hours. Faster than that, and you risk osmotic demyelination syndrome-a rare but devastating brain injury.

For hypernatremia: Rehydrate slowly with water or IV fluids. The goal is to bring sodium down by no more than 10 mmol/L in 24 hours. Use plain water or half-normal saline. Avoid sugary drinks or salty broths.

Diuretics need careful review. Loop diuretics (like furosemide) are safer than thiazides in advanced CKD because thiazides don’t work well when GFR is low-and they increase hyponatremia risk. The FDA warns against using thiazides in patients with eGFR under 30. Vasopressin blockers (vaptans) are off-limits in advanced CKD because your kidneys can’t respond to them.

Why Diet Alone Isn’t Enough

Patients are often told to follow a “low-sodium, low-potassium, low-protein” diet. But these goals conflict. Cutting protein reduces urea, which your kidneys use to draw water out. Less urea means less water excretion-worsening hyponatremia. Cutting sodium too hard can trigger salt-wasting, especially in older adults.

A 2020 study found patients need 3-6 sessions with a renal dietitian just to understand what to eat. Many think “no salt” means no flavor, so they avoid all processed foods and even table salt-but end up drinking too much water, thinking it’s healthy. That’s a recipe for hyponatremia.

Instead of blanket restrictions, focus on balance. Eat enough protein to maintain muscle, keep sodium at moderate levels (2-3 grams/day), and drink only when thirsty-not because you’re told to “drink 8 glasses.”

The New Tools Changing the Game

In 2023, the FDA approved a wearable sodium monitoring patch for CKD patients. It measures interstitial sodium continuously and correlates with blood levels at 85% accuracy. This means doctors can see sodium trends over days, not just one lab test. It’s especially helpful for catching early drops before symptoms show.

Guidelines are also changing. The 2024 KDIGO conference is pushing for individualized fluid targets based on your remaining kidney function-not a fixed number for everyone. Research is also exploring the gut-kidney axis: early data suggests your intestines might help handle sodium when kidneys fail, opening doors for new therapies.

What You Can Do Today

If you have CKD:

• Ask your nephrologist for a sodium blood test every 3-6 months-even if you feel fine.
• Keep a daily fluid log. Note how much you drink and any swelling or weight gain.
• Don’t restrict sodium unless your doctor says so. Avoid extreme low-sodium diets.
• Review all your medications. Thiazide diuretics are risky in advanced CKD.
• Work with a renal dietitian. They can help you balance sodium, protein, and fluid without harming you.
• Watch for symptoms: confusion, nausea, muscle cramps, dizziness, or sudden weight gain.

Hyponatremia and hypernatremia aren’t inevitable in CKD. But they’re easy to miss-and deadly if ignored. The key isn’t just treating the number-it’s understanding how your kidneys, your diet, and your body’s water balance all fit together.

Frequently Asked Questions

What Comes Next

If you’re managing CKD, sodium balance is one of the most overlooked but critical parts of your care. Don’t wait for symptoms to appear. Ask for regular sodium tests, track your fluid intake, and work with your care team to find the right balance-not the strictest diet. The goal isn’t to eliminate sodium or water-it’s to help your kidneys do their job without pushing your body past its limits.