High sodium levels in the blood, called hypernatremia, almost always result from losing too much water rather than consuming too much salt. Normal blood sodium falls between 135 and 145 mEq/L, and anything above 145 mEq/L is considered elevated. Because sodium concentration is really a ratio of sodium to water, anything that tips that balance (less water, more sodium, or both) can push levels up.
Water Loss Is the Most Common Cause
The single biggest driver of high sodium is unreplaced water loss. Your body loses water constantly through sweat, breathing, urine, vomiting, and diarrhea. Normally, thirst kicks in and you drink enough to keep up. Problems start when water leaves the body faster than it’s replaced, or when something prevents you from drinking enough. The sodium itself hasn’t changed much; there’s simply less water to dilute it.
This water loss can happen through the kidneys or through other routes. Fever, heavy sweating, severe burns, and prolonged diarrhea all pull water out without taking much sodium along with it. On the kidney side, certain conditions force the kidneys to dump large volumes of dilute urine, which concentrates the sodium left behind in the bloodstream.
Osmotic Diuresis: When Other Substances Pull Water Out
High blood sugar is one of the more common triggers. When glucose spills into the urine, it drags water along with it, a process called osmotic diuresis. The same thing happens when urea (a waste product from protein metabolism) builds up and is excreted in large amounts. In both cases, the kidneys excrete proportionally more water than sodium, so blood sodium climbs. This is why people with poorly controlled diabetes or those on high-protein tube feeds can develop hypernatremia if fluid intake doesn’t keep pace.
Diabetes Insipidus
Diabetes insipidus (DI) has nothing to do with blood sugar. It’s a condition where the kidneys can’t hold onto water properly, producing enormous volumes of very dilute urine, sometimes several liters a day. Without enough fluid replacement, sodium levels rise quickly.
There are two forms. In central DI, the brain doesn’t produce enough of the hormone (ADH) that tells the kidneys to conserve water. This can result from head trauma, brain surgery, tumors, or certain genetic conditions. In nephrogenic DI, the brain sends the signal just fine, but the kidneys don’t respond to it. Lithium is a well-known cause of this form, but it can also be triggered by kidney disease, certain antibiotics, and genetic disorders. Both types produce the same result: excessive water loss and, if fluids aren’t replaced, rising sodium.
Medications That Raise Sodium
A surprisingly long list of medications can push sodium levels up, mostly by causing the kidneys to lose extra water. Lithium is the most widely recognized culprit, causing a form of nephrogenic diabetes insipidus in a significant number of people who take it long-term. Loop diuretics (commonly prescribed for heart failure and high blood pressure) increase urine output and can tip the balance if fluid intake doesn’t match. Corticosteroids also have a mild diuretic effect that contributes.
Other medications linked to water loss through the kidneys include certain antibiotics (aminoglycosides, some penicillin derivatives), antiviral drugs, the antifungal amphotericin B, and some chemotherapy agents. Even osmotic laxatives like lactulose and sorbitol can cause enough water loss through the gut to affect sodium levels. Phenytoin, a seizure medication, can trigger central diabetes insipidus. Alcohol also suppresses ADH, which is part of why heavy drinking leads to dehydration.
Too Much Sodium Coming In
Pure sodium overload is a less common cause, but it does happen. It’s almost always tied to a medical setting or an accident rather than eating salty food. Examples include receiving large amounts of sodium bicarbonate during resuscitation, surgical irrigation with concentrated salt solutions, or dialysis errors. Improperly mixed infant formula is another recognized cause in newborns. Intentional salt ingestion, whether from a dare or in cases of abuse, has also been documented.
For most healthy adults, eating a high-sodium diet won’t push blood sodium above 145 mEq/L. Healthy kidneys and an intact thirst mechanism are remarkably good at maintaining balance. The body simply makes you thirsty and excretes the excess. High sodium levels from diet alone typically require a simultaneous inability to access or drink water.
Who Is Most Vulnerable
Older adults are at the highest risk. Aging blunts the thirst response, so the natural signal to drink doesn’t fire as strongly. Cognitive decline, dementia, or physical limitations that make it hard to get a glass of water compound the problem. Many older adults also take diuretics or other medications that increase water loss. In hospital settings, hypernatremia is disproportionately common among elderly and critically ill patients.
Infants are the other high-risk group, particularly premature babies. About 40% of preterm infants develop hypernatremia, largely because their immature kidneys can’t concentrate urine well, and they lose water rapidly through their thin skin. In full-term newborns, the most common trigger is inadequate breastfeeding, often from latching difficulties or insufficient milk production, especially after early hospital discharge without enough lactation support. Improperly prepared formula with too much powder and not enough water is another preventable cause.
What High Sodium Does to the Brain
The most dangerous effect of hypernatremia is on brain cells. When sodium in the blood rises sharply, water gets pulled out of brain tissue by osmosis, causing cells to shrink. This shrinkage is what produces the neurological symptoms: confusion, irritability, muscle twitching, and in severe cases, seizures or coma.
The speed of onset matters enormously. Acute hypernatremia, developing over hours, carries a mortality rate of roughly 50 to 60%. When sodium climbs above 160 mEq/L, mortality reaches about 75%. Chronic hypernatremia, developing over days, is somewhat better tolerated because brain cells have time to adapt by pulling in small molecules to retain water. But even chronic cases are dangerous if left untreated.
Severity by the Numbers
Clinicians classify hypernatremia by how far sodium has risen above 145 mEq/L:
- Mild: 146 to 150 mEq/L
- Moderate: 151 to 155 mEq/L
- Severe: above 155 mEq/L
- Extreme: above 190 mEq/L
Mild elevations may cause only subtle symptoms like increased thirst and slightly decreased energy. Moderate and severe levels bring more obvious neurological changes: lethargy, weakness, muscle pain, and confusion. Correction has to happen carefully, because lowering sodium too fast can cause brain swelling, which is its own emergency. The underlying cause determines the treatment approach, whether that’s replacing fluids, adjusting medications, or managing the condition driving the water loss.