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

When your kidneys aren’t working right, even small changes in sodium 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). About 1 in 5 people with advanced CKD will develop one of these sodium disorders. And the worst part? Many of them are preventable-if you know what to look for.

Why Your Kidneys Control Your Sodium

Your kidneys don’t just filter waste. They’re the main regulators of sodium and water in your body. Healthy kidneys adjust urine concentration to keep sodium levels steady, even when you eat salty food or drink too much water. But when CKD progresses, this system breaks down. By stage 4 or 5 (GFR below 30 mL/min/1.73m²), your kidneys can’t make concentrated or dilute urine properly. That means they can’t handle extra water or salt like they used to.

This is why even small changes in fluid intake can throw sodium levels out of balance. Someone with early CKD might need to pee out 3 liters of water to get rid of a normal salt load. But someone with late-stage CKD? They might not be able to pee out more than 800 mL a day. That’s why drinking an extra bottle of water could push sodium levels dangerously low.

Hyponatremia: The Silent Threat in CKD

Hyponatremia-serum sodium below 135 mmol/L-is the most common sodium disorder in CKD. It shows up in 60-65% of cases, mostly as euvolemic hyponatremia, meaning your total body water is high, but your sodium level is low. This isn’t caused by dehydration. It’s caused by your kidneys losing the ability to flush out excess water.

Thiazide diuretics, often used for high blood pressure, make this worse. They’re effective early in CKD but become useless once GFR drops below 30. Yet many patients still get them, and that’s a big reason why hyponatremia keeps happening. In fact, thiazides are linked to 25-30% of euvolemic hyponatremia cases in CKD patients.

But here’s the twist: dietary advice meant to help can hurt. Doctors often tell CKD patients to cut back on sodium, potassium, and protein to protect the kidneys. But when you restrict too much, your body can’t make enough solute to excrete water. That leads to water retention and hyponatremia. A 2023 Japanese study found that patients on strict solute-restricted diets had higher rates of hyponatremia-even when they drank less water.

The risks aren’t theoretical. People with hyponatremia are 1.94 times more likely to die than those with normal sodium. They’re more likely to fall, break bones, or develop cognitive decline. In hospitals, hyponatremia increases death risk by 28%. And if it develops while you’re already hospitalized? The risk jumps even higher.

Hypernatremia: The Overlooked Danger

Hypernatremia-sodium above 145 mmol/L-is less common but just as dangerous. It happens when your body loses too much water and you can’t replace it. In CKD, this often occurs in older adults who don’t feel thirsty, or those with dementia who can’t ask for water. It can also happen if you’re on diuretics and don’t drink enough, or if you have a fever or are in a hot climate.

People with advanced CKD are especially vulnerable because their kidneys can’t concentrate urine. Even if they drink water, their body can’t hold onto it. And if they’re on dialysis, fluid restrictions can make it worse. If you’re not drinking enough and your kidneys can’t conserve water, sodium builds up in your blood.

Correcting hypernatremia too fast is risky. Lowering sodium by more than 10 mmol/L in 24 hours can cause brain swelling, leading to seizures or coma. Treatment isn’t about giving IV salt-it’s about giving water slowly. Oral fluids are preferred. If you can’t drink, IV hypotonic fluids are used, but only under strict monitoring.

How Sodium Disorders Are Classified in CKD

Doctors classify hyponatremia by volume status:

• Hypovolemic (15-20%): You’ve lost both sodium and water, but more sodium. This happens with diuretics, vomiting, or rare salt-wasting syndromes like milk-alkali syndrome.
• Euvolemic (60-65%): Normal volume, but too much water. This is the most common type in CKD. Your body holds water because your kidneys can’t get rid of it.
• Hypervolemic (15-20%): You have too much total fluid, often with swelling in the legs or lungs. This happens in late-stage CKD with heart failure or nephrotic syndrome.

Knowing the type helps guide treatment. Giving fluids to someone with hypervolemic hyponatremia? That’s dangerous. Restricting fluids in someone with hypovolemic hyponatremia? That’s deadly.

Treatment: What Works and What Doesn’t

Fluid restriction is the first step for hyponatremia in CKD. But it’s not one-size-fits-all:

• Early CKD: 1,000-1,500 mL/day
• Advanced CKD: 800-1,000 mL/day

That’s less than a standard water bottle per hour. Many patients find this hard. And if they’re also told to limit protein and potassium? It becomes overwhelming. That’s why dietitians spend 3-6 sessions teaching patients how to balance these restrictions.

Sodium supplements? Rarely needed. But in salt-wasting syndromes (5-8% of advanced CKD), patients may need 4-8 grams of sodium chloride daily. That’s about 1-2 teaspoons of salt. But giving salt without checking volume status can cause heart failure or lung fluid buildup.

Drugs like vaptans (vasopressin blockers) are off-limits in advanced CKD. They rely on kidney function to work-and in stage 4 or 5, the kidneys can’t respond. Loop diuretics like furosemide are preferred over thiazides because they still work when GFR is low.

And correction speed matters. Never raise sodium by more than 6 mmol/L in 24 hours, and never more than 8 mmol/L total in a day. Go too fast? You risk osmotic demyelination syndrome-a rare but devastating brain injury that can leave you locked-in, unable to speak or move.

What’s New in 2025

A new sodium monitoring patch, approved by the FDA in 2023, measures sodium levels in the skin continuously. It’s not a replacement for blood tests, but it gives early warnings. In trials, it matched serum sodium levels 85% of the time. For CKD patients on strict fluid limits, this could mean fewer ER visits.

Guidelines are changing too. The 2024 KDIGO conference is pushing for individualized fluid targets based on how much kidney function remains-not just a fixed number. And research is exploring the gut-kidney connection: early CKD may trigger the intestines to absorb more sodium to make up for kidney loss.

Real-Life Challenges

Managing sodium in CKD isn’t just medical-it’s personal. Most patients with advanced CKD are over 65. Many have memory problems, arthritis, or live alone. They might forget to drink. Or they might drink too much because they’re thirsty and don’t know the limits. Family members often misunderstand “low-sodium” as “no sodium,” leading to extreme restriction that triggers hyponatremia.

Medications add another layer. A blood pressure pill, a heart medication, an antibiotic-all can interfere with sodium balance. Pharmacists need to be part of the care team.

Studies show that when nephrologists, dietitians, and pharmacists work together, hospitalizations for sodium disorders drop by 35%. That’s not just a statistic-it’s fewer emergency trips, fewer falls, fewer days in the hospital.

What You Can Do

If you or someone you care for has CKD:

• Know your GFR. Ask your doctor what stage you’re in.
• Track your fluid intake. Use a marked water bottle or app.
• Don’t assume “low-sodium” means “no salt.” Talk to a renal dietitian.
• Watch for symptoms: confusion, nausea, headaches, weakness, swelling, or dry mouth.
• Ask about your medications. Are any of them thiazide diuretics? If you’re in stage 4 or 5, they may need to change.
• Get a sodium monitoring patch if available-it’s not covered everywhere yet, but it’s worth asking about.

There’s no magic fix. But understanding how sodium and kidneys interact gives you power. You don’t have to guess. You can act-before the numbers turn dangerous.