The most common cause of hypernatremia is losing more water than sodium from the body. This imbalance raises the concentration of sodium in your blood above the normal range, which is typically defined as a serum sodium level above 145 mmol/L. While excess sodium intake can also drive levels up, the vast majority of cases stem from a water deficit, not from taking in too much salt.
Why Water Loss Is the Primary Driver
Your body maintains sodium concentration within a narrow range by balancing water intake with water loss. When you lose water faster than you replace it, the sodium already in your blood becomes more concentrated. This is the fundamental mechanism behind most hypernatremia cases.
The water loss can happen through several routes. The most straightforward are sweating, breathing (especially with fever or rapid breathing), and gastrointestinal losses like diarrhea and vomiting. In each of these situations, the fluid leaving your body contains less sodium than your blood does, so the net effect is a rise in sodium concentration. Osmotic diarrhea, caused by poorly absorbed substances like lactulose, sorbitol, or lactose in people who are lactose-intolerant, is a particularly notable cause because it pulls large volumes of water into the intestines. In severe cases like cholera, where stool volume exceeds three liters, hypernatremia along with dangerous potassium loss and acid buildup can develop rapidly.
Your kidneys normally compensate by holding onto water and producing small amounts of concentrated urine. When the water loss is coming from outside the kidneys (vomiting, diarrhea, sweating), you’ll typically see low urine volume with high concentration. That’s your body doing its job. But if the loss outpaces what the kidneys can save, sodium levels climb anyway.
When the Kidneys Can’t Conserve Water
Sometimes the kidneys themselves are the source of the problem. In diabetes insipidus, the kidneys lose the ability to concentrate urine and instead excrete large volumes of dilute fluid. This happens in one of two ways. In the central form, the brain doesn’t produce enough antidiuretic hormone (ADH), the signal that tells the kidneys to reabsorb water. In the nephrogenic form, the kidneys can’t respond to ADH even when levels are normal. Either way, the result is the same: you urinate excessively, and if you don’t drink enough to keep up, sodium levels rise.
A key diagnostic clue is what the urine looks like. In diabetes insipidus, urine is inappropriately dilute relative to the blood, with normal or even high urine volume. That’s the opposite of what you’d expect when the body is dehydrated, and it signals that the kidneys aren’t doing their part to conserve water. Symptoms of the resulting dehydration include weakness, lethargy, fatigue, and muscle aches.
Older Adults Are Especially Vulnerable
Hypernatremia disproportionately affects older adults, particularly those in nursing homes or with cognitive impairment. The prevalence among community-dwelling older adults may reach 3.7%, depending on the diagnostic threshold used. Two factors converge to make this group high-risk: the thirst mechanism weakens with age, and many older adults have limited physical access to water. Someone with dementia, mobility limitations, or who depends on caregivers for fluids may simply not drink enough to keep pace with normal daily water losses.
In nursing home settings, hypernatremia and dehydration are sometimes considered indicators of neglect. But there are also unavoidable causes, including infections with fever, medications that increase urine output, and conditions that impair swallowing. Recognizing which factors are at play matters for both care and accountability.
Hospital-Acquired Hypernatremia
Developing hypernatremia while already in the hospital is more common than you might expect, and it’s largely considered preventable. In one observational study of general medical inpatients, 88% of patients were receiving intravenous fluids before their sodium crossed into the moderately high range, and normal saline (0.9% sodium chloride) was the predominant fluid being given. Even after sodium levels started climbing, over one-third of the IV fluids prescribed were still normal saline rather than lower-sodium alternatives.
Other hospital-related contributors include fasting periods before procedures, delirium that prevents patients from asking for water, diuretic medications, tube feeding formulas that don’t contain enough free water, and thickened fluid regimens for patients with swallowing difficulties. The controlled nature of a hospital setting means clinicians have the ability to monitor and prevent these imbalances, which is why hospital-acquired hypernatremia is often viewed as an iatrogenic problem.
Excess Sodium Intake Is Rare but Real
Pure sodium overload accounts for a much smaller share of hypernatremia cases and is most often seen in medical settings. Causes include excessive sodium bicarbonate given during resuscitation, improperly mixed infant formula, accidental or intentional ingestion of salt tablets, and, rarely, seawater drowning. Certain hormonal conditions like hyperaldosteronism, where the adrenal glands produce too much of a hormone that causes the kidneys to retain sodium, can also contribute.
Severity Levels and Why They Matter
Hypernatremia is classified by how high the sodium level goes. Mild cases fall between 146 and 150 mmol/L, moderate between 151 and 155, and severe above 155. Extreme cases above 190 mmol/L have been documented but are rare. The distinction matters because the risk of serious complications, including brain shrinkage and neurological damage, increases with severity.
Mortality rates for severe hypernatremia are strikingly high. In a large study of patients with severe cases, 30-day mortality reached roughly 50% among those whose sodium was corrected slowly, compared to about 32% in those corrected more quickly. At one year, those numbers rose to 73% and 52%, respectively. These figures partly reflect the fact that severe hypernatremia tends to occur in people who are already critically ill, but the sodium imbalance itself adds independent risk through its effects on the brain and other organs.
How the Body Signals Trouble
Mild hypernatremia often starts with increased thirst, dry mouth, and decreased urine output. As levels rise, symptoms shift toward the neurological: restlessness, irritability, and confusion. In severe cases, you may see muscle twitching, seizures, or loss of consciousness. The danger is that the people most vulnerable to hypernatremia, particularly older adults with impaired thirst or cognition, are the least likely to notice or report early warning signs.
The speed of onset also shapes the symptoms. When sodium rises quickly over hours, the brain hasn’t had time to adapt, and neurological symptoms tend to be more dramatic. When it develops gradually over days, the brain compensates by generating internal solutes to retain water, which blunts the symptoms but creates a different risk: correcting the sodium too quickly can then cause the brain to swell.