High blood sodium, called hypernatremia, almost always comes down to one thing: your body has too little water relative to the amount of sodium in your blood. A normal blood sodium level sits below 145 mmol/L. Anything above that threshold is considered elevated, with levels between 145 and 150 classified as mild, 151 to 155 as moderate, and above 155 as severe.
The surprise for most people is that high sodium rarely comes from eating too much salt. In a healthy person with normal thirst and access to water, the body is remarkably good at balancing things out. High sodium typically develops when something disrupts that balance, either by causing you to lose more water than sodium or by preventing you from drinking enough to keep up.
Water Loss Is the Most Common Cause
The single biggest driver of high sodium is losing more water than you’re replacing. This can happen through your skin, your gut, or your kidneys, and the underlying trigger matters because it determines how your body responds.
Sweating, fever, and burns. Heavy sweating from exercise, prolonged heat exposure, or a high fever pulls water out of your body faster than sodium. Severe burns do the same thing by damaging the skin’s ability to hold in fluid. In all of these cases, sweat and fluid loss are “hypotonic,” meaning they contain less sodium than your blood does. So your blood sodium concentration rises as water leaves.
Vomiting and diarrhea. Gastroenteritis and prolonged vomiting cause you to lose both water and sodium, but water losses typically outpace sodium losses. If you’re too nauseated to drink or a young child can’t communicate thirst, sodium climbs quickly.
Not drinking enough water. This sounds simple, but it’s one of the most important factors. Your brain has a thirst center that normally kicks in well before sodium rises to dangerous levels. As long as you can feel thirst and act on it, even large water losses can be corrected by drinking. Sustained high sodium almost always involves some barrier to drinking, whether that’s impaired thirst sensation, physical inability to reach water, or altered consciousness.
Kidney Problems That Drive Sodium Up
Your kidneys are your main tool for regulating sodium. They decide how much water to hold onto and how much to let go in urine. When they malfunction, you can lose enormous amounts of water without losing much sodium.
A condition called diabetes insipidus (now formally known as arginine vasopressin disorder) is one of the clearest examples. It comes in two forms. In the central form, the brain doesn’t produce enough of the hormone that tells your kidneys to conserve water. This can happen after head trauma, brain surgery, or from tumors or inflammatory diseases affecting the pituitary gland. In the nephrogenic form, the kidneys simply don’t respond to that hormone, even when levels are normal. The result in both cases is the same: you produce large volumes of very dilute urine, and your blood sodium rises if you can’t drink enough to keep pace.
Interestingly, many people with diabetes insipidus maintain near-normal sodium levels for years because their thirst drive compensates. Sodium only spikes when thirst is also impaired or when illness, surgery, or sedation prevents adequate drinking.
Osmotic diuresis is another kidney-related cause. When blood sugar runs very high in uncontrolled diabetes, the excess glucose spills into the urine and drags water along with it. The same thing happens with certain medical treatments that pull fluid through the kidneys. The net effect is heavy water loss through urine, concentrating sodium in the blood.
Medications That Raise Sodium
Several classes of drugs can push sodium levels up, usually by interfering with the kidneys’ ability to concentrate urine.
- Lithium is one of the most well-known culprits. Used for mood disorders, it can damage the kidneys’ ability to respond to the water-conserving hormone, causing a form of nephrogenic diabetes insipidus.
- Loop diuretics (water pills) increase urine output. Because the fluid lost is relatively dilute compared to blood, sodium concentration can rise.
- Certain antibiotics and antifungals, including some used to treat serious infections, can impair the kidneys’ concentrating ability or cause mineral imbalances that indirectly reduce water retention.
Drug-induced high sodium is more common in hospital settings, where patients may also have limited ability to drink freely. But it can happen at home, particularly in older adults on multiple medications who aren’t drinking enough fluid.
Can Eating Too Much Salt Cause It?
In theory, yes. In practice, it’s rare in healthy adults. Your kidneys and thirst mechanism are efficient enough to handle a salty meal by making you thirsty and adjusting how much sodium you excrete. Severe hypernatremia from salt intake alone has been documented, but it typically involves extreme circumstances: accidental ingestion of large amounts of table salt, improperly mixed infant formula, or intentional salt poisoning. The estimated lethal dose is roughly 1 gram of salt per kilogram of body weight, which would raise blood sodium by about 30 mmol/L. That’s far beyond what normal eating produces.
The key distinction is that dietary salt becomes dangerous mainly when it’s combined with an inability to drink water or with another condition that already impairs water balance, like untreated hypothyroidism or kidney disease.
Why Older Adults and Infants Are Most Vulnerable
High sodium disproportionately affects people at the extremes of age, and the reasons are straightforward.
Older adults experience a well-documented decline in thirst sensation. Even when mildly dehydrated, they may not feel compelled to drink. This becomes especially dangerous for hospitalized patients or nursing home residents who depend on others to bring them water. Frail elderly people who develop a fever, diarrhea, or a urinary tract infection can tip into hypernatremia within hours if fluid intake doesn’t keep up with losses.
Infants face a different version of the same problem: they can’t get water on their own. Breastfed newborns are at risk if milk supply is inadequate and the problem isn’t recognized quickly. Their small body size means even modest water deficits produce significant changes in sodium concentration.
How High Sodium Affects Your Body
When sodium levels rise, water moves out of your cells to try to dilute the blood. Brain cells are particularly sensitive to this shift. Mild elevations may cause intense thirst, fatigue, and irritability. As levels climb higher, symptoms progress to confusion, muscle twitching, and lethargy. Severe hypernatremia (above 155 mmol/L) can cause seizures, loss of consciousness, and permanent brain damage.
The speed of onset matters as much as the absolute number. A sodium level that rises rapidly over hours is more dangerous than one that creeps up over days, because the brain has less time to adapt. Brain cells can gradually adjust their internal chemistry to tolerate higher sodium, but that process takes 24 to 48 hours. This is also why correcting high sodium too quickly can itself be harmful: once the brain has adapted, a sudden drop in sodium causes water to rush back into brain cells and swell them.
How It’s Diagnosed and Treated
A basic blood test (a metabolic panel) reveals high sodium. The harder part is figuring out why it’s elevated. Doctors typically check how concentrated your urine is. If your kidneys are working properly and you’re dehydrated, your urine should be very concentrated as your body tries to conserve water. If your urine is dilute despite high blood sodium, that points toward diabetes insipidus or a kidney problem that’s preventing normal water conservation.
Treatment centers on replacing the missing water, either by drinking fluids or receiving them intravenously if you can’t drink. The correction is done gradually, typically lowering sodium by no more than 10 to 12 mmol/L per day, to protect the brain from swelling. If a specific cause like a medication or uncontrolled blood sugar is identified, addressing that underlying problem is equally important.