High sodium levels in the blood, called hypernatremia, are overwhelmingly caused by not taking in enough water rather than by consuming too much salt. In elderly adults, this is especially common because aging changes both the brain’s thirst signals and the kidneys’ ability to conserve water. Normal blood sodium falls between 136 and 145 mEq/L, and levels above 145 signal a problem that can become dangerous quickly, particularly in older adults.
Why Aging Itself Raises the Risk
Several changes happen in the body with age that make high sodium far more likely. The most important is a decline in thirst. Older adults simply don’t feel as thirsty as younger people do, even when their bodies are already running low on water. This blunted thirst response means dehydration can set in well before the person thinks to reach for a glass of water.
At the same time, the kidneys lose their ability to concentrate urine efficiently. Kidney mass shrinks, blood flow to the kidneys drops, and the filtering rate slows. Even though the body still produces the hormone that tells the kidneys to hold onto water (ADH), the aging kidney becomes less responsive to it. The result is that the kidneys let more water escape into the urine than they should. When you combine reduced water intake with increased water loss through the kidneys, sodium concentration in the blood rises.
These two changes, diminished thirst and reduced kidney function, are present to some degree in most people over 65. They form the background vulnerability that makes every other cause on this list more dangerous for older adults than it would be for someone younger.
Acute Illness and Infection
Infections are one of the most common triggers for high sodium in elderly people living at home or in care facilities. A urinary tract infection, pneumonia, or other acute illness often brings fever, which increases water loss through the skin and breathing. At the same time, the person may feel too unwell to eat or drink normally. Community-acquired hypernatremia most often develops in elderly people who are mentally and physically impaired and have an active infection layered on top of already limited fluid intake.
Vomiting and diarrhea from a stomach bug or food poisoning can also drive sodium up fast. Both pull water and electrolytes out of the body, and if the person can’t replace fluids quickly enough, sodium concentrates in the blood within hours.
Limited Access to Water
Physical and cognitive limitations play a surprisingly large role. Someone with advanced dementia may not remember to drink. A person recovering from a stroke or living with severe arthritis may not be able to get up and pour water independently. In hospitals and nursing homes, patients who rely on staff for fluids are particularly vulnerable if hydration isn’t actively monitored.
This is not a minor issue. Research from long-term care settings has found that subclinical, chronic underhydration is a common finding in elderly nursing home residents regardless of whether they eat by mouth or receive tube feeding. One study found that 75% of patients who were eating orally but had swallowing difficulties showed markers of dehydration. Even among patients receiving nutrition through a feeding tube, 18% were dehydrated, a finding the researchers called “surprising” given the assumption that tube-fed patients should be adequately hydrated.
Medications That Shift Fluid Balance
Several common medications can tip the balance toward high sodium. Loop diuretics (often prescribed for heart failure or high blood pressure) increase urine output, pulling water from the body faster than sodium. Certain drugs can also interfere with the hormone system that regulates water retention, making the kidneys less effective at holding onto fluid. Nonsteroidal anti-inflammatory drugs like ibuprofen cause sodium retention in the body, which can contribute to the problem from the opposite direction.
Older adults often take multiple medications at once, and the combined effect on fluid balance can be hard to predict. A medication that causes no problems on its own may become an issue when paired with another drug, a hot day, or a mild stomach illness.
Tube Feeding and High-Protein Diets
Tube feeding formulas are calorie-dense and often high in protein. When protein is broken down in the body, it produces waste products that the kidneys need extra water to flush out. If the feeding plan doesn’t include enough free water flushes between formula doses, sodium gradually climbs. Nutrition teams in hospitals and care facilities typically calculate free water needs alongside caloric goals, but errors or oversights happen, especially during transitions between care settings.
For sodium levels under 150 mEq/L in a tube-fed patient, additional water given through the tube in divided doses (spread throughout the day) is often enough to correct the problem. Once sodium climbs above 150, the situation usually calls for intravenous fluids administered carefully to avoid correcting too quickly, which carries its own risks.
Underlying Medical Conditions
Uncontrolled diabetes is a frequent contributor. When blood sugar runs very high, the body tries to flush excess glucose through the urine, pulling large volumes of water along with it. This osmotic water loss can raise sodium levels significantly. Diabetes insipidus, a separate condition where the kidneys produce abnormally large amounts of dilute urine, can also cause hypernatremia. It occurs either because the brain doesn’t produce enough of the water-retention hormone or because the kidneys stop responding to it, and both forms become more problematic with age as baseline kidney function is already reduced.
Chronic kidney disease, which affects a large share of people over 70, further limits the kidneys’ already diminished ability to regulate water and sodium. Heart failure can also complicate fluid balance, both through the disease itself and through the diuretics used to manage it.
How High Sodium Affects the Brain
The symptoms of high sodium are mostly neurological, and they can be easy to miss in an elderly person who already has some cognitive decline. When sodium rises in the blood, water gets pulled out of brain cells by osmosis, causing them to shrink. In the acute setting, even modest elevations in sodium can produce severe neurological symptoms because of this cellular shrinkage.
Early signs include increased thirst (though this signal is often muted in older adults), restlessness, and irritability. As levels climb, confusion, excessive sleepiness, and muscle twitching develop. Severe cases can progress to seizures or loss of consciousness. Because symptoms like confusion and lethargy overlap with so many other conditions common in elderly people, hypernatremia is frequently caught on blood work rather than recognized by symptoms alone.
Why It’s Particularly Dangerous in Older Adults
High sodium carries real mortality risk, especially when it occurs alongside other serious illness. A study of elderly ICU patients (age 65 and older) found that those with elevated sodium who also had a severe infection had a 7-day mortality rate roughly double that of patients with high sodium but no infection. When sodium peaked above approximately 155 mEq/L, outcomes worsened dramatically: the 28-day mortality rate jumped to about 76% in that group compared to 37% in those whose sodium stayed below that threshold. Median survival time was significantly shorter in the higher-sodium group.
Even outside the ICU, hypernatremia in older adults is associated with longer hospital stays, higher rates of complications, and greater likelihood of not returning to baseline function. The condition itself causes harm, but it also serves as a marker that something has gone significantly wrong with the body’s ability to maintain balance.
Practical Steps for Prevention
Because thirst is an unreliable signal in older adults, fluid intake needs to be intentional rather than driven by feeling thirsty. Keeping water within arm’s reach, offering fluids at regular intervals throughout the day, and tracking intake loosely (aiming for roughly six to eight cups daily unless fluid-restricted for another condition) all help. Foods with high water content, like soups, watermelon, cucumbers, and yogurt, contribute meaningfully to hydration.
During illness, hot weather, or any episode of vomiting or diarrhea, fluid needs increase and should be addressed proactively. If you’re caring for someone who has difficulty swallowing, limited mobility, or cognitive impairment, checking in on fluid intake several times a day makes a measurable difference. For anyone on diuretics or multiple medications, periodic blood work to check electrolytes catches rising sodium before symptoms develop.