High sodium levels in the blood, a condition called hypernatremia, almost always come down to one of two problems: your body lost too much water or it took in too much sodium. Normal blood sodium falls between 136 and 145 mmol/L, and anything above 145 mmol/L qualifies as elevated. Water loss is by far the more common cause, but the specific triggers range from everyday dehydration to serious medical conditions.
Water Loss Is the Most Common Cause
Sodium concentration in your blood is really a ratio: the amount of sodium relative to the amount of water. When you lose water without replacing it, that ratio shifts and sodium levels climb. This happens through several routes.
Your skin and lungs constantly lose small amounts of water through sweating and breathing. Normally this is insignificant, but fever, heavy sweating, burns, and rapid breathing can dramatically increase these losses. Gastrointestinal problems are another major route. Prolonged diarrhea, repeated vomiting, or drainage through surgical tubes can pull large volumes of fluid from the body faster than you replace them.
The kidneys themselves can also be the source of excess water loss. Loop diuretics (a common class of “water pills”) cause the kidneys to flush out dilute urine, taking more water than sodium with it. Osmotic diuresis is another kidney-driven mechanism: when substances like glucose, urea, or the medication mannitol build up in the urine, they drag extra water along with them. High blood sugar in uncontrolled diabetes is one of the most frequent triggers for this type of water loss.
Diabetes Insipidus and the Kidneys
Diabetes insipidus is a condition where the kidneys produce large volumes of very dilute urine, sometimes several liters a day, because the water-conservation system breaks down. It has nothing to do with blood sugar despite sharing a name with diabetes mellitus. There are two forms.
In the central form, the brain doesn’t produce enough of the hormone that tells the kidneys to hold onto water (antidiuretic hormone, or ADH). This can result from head trauma, brain surgery, tumors, or other damage to the pituitary gland. In the nephrogenic form, the brain produces ADH normally, but the kidneys don’t respond to it. Both forms lead to massive water loss through urine and, if fluid intake can’t keep up, rising sodium levels.
Medications That Raise Sodium
A number of drugs can push sodium levels up, usually by interfering with the kidneys’ ability to concentrate urine. Lithium, widely used for bipolar disorder, is the most common culprit. Nearly 50% of people on long-term lithium therapy develop some degree of nephrogenic diabetes insipidus, meaning their kidneys stop responding properly to the water-conservation hormone.
Other medications linked to elevated sodium include:
- Amphotericin B, an antifungal drug that can damage kidney tubules
- Phenytoin and alcohol, which temporarily suppress the brain’s release of ADH
- Demeclocycline, an antibiotic that can cause nephrogenic diabetes insipidus in nearly all patients at higher doses
- Corticosteroids and high-protein supplements, which increase urea production and drive osmotic water loss
- Lactulose and sorbitol, osmotic laxatives that pull water into the gut and out of the body through diarrhea
Too Much Sodium Intake
Less commonly, sodium levels spike because the body takes in a large load of sodium all at once. This is typically rapid in onset, developing within minutes to hours. Documented cases include accidental ingestion of large amounts of salt or soy sauce, drinking seawater, and medical errors involving concentrated sodium solutions like sodium bicarbonate infusions or dialysis fluid mix-ups. These scenarios are rare, but they produce some of the most dangerously high sodium readings recorded in clinical literature.
Impaired Thirst and Limited Water Access
Your body’s main defense against rising sodium is thirst. When sodium starts climbing, you feel thirsty, drink water, and the balance corrects itself. The system works well unless something disrupts it.
Age is the single greatest risk factor for high sodium. People older than 65 are especially vulnerable because the thirst sensation naturally weakens with age, a phenomenon sometimes called geriatric hypodipsia. Dementia, stroke, and other neurological conditions can damage the brain’s thirst centers directly or leave a person unable to communicate that they’re thirsty. Physical disability, sedation, and post-surgical states create the same problem from a different angle: the person may feel thirsty but can’t get to water independently. This is why hypernatremia disproportionately affects hospitalized patients, nursing home residents, and people with cognitive impairment.
Rarer causes of impaired thirst include tumors or granulomatous diseases affecting the hypothalamus, the brain region that monitors sodium concentration and generates the thirst signal.
What High Sodium Does to the Body
When sodium levels rise, water gets pulled out of cells throughout the body to try to dilute the blood. Brain cells are particularly sensitive to this shrinkage. Early symptoms tend to be nonspecific: loss of appetite, muscle weakness, restlessness, nausea, and vomiting. As levels climb higher, neurological symptoms dominate. Lethargy, confusion, irritability, and eventually stupor or coma can develop, especially when the increase is rapid.
At its most dangerous, the shrinking of brain tissue can tear blood vessels, causing cerebral bleeding or subarachnoid hemorrhage. In adults, sodium concentrations above 160 mmol/L carry a 75% mortality rate. Even when corrected, severe cases can leave permanent neurological damage.
How It’s Corrected
Treatment centers on slowly replacing lost water, either by mouth or intravenously. The key word is slowly. Dropping sodium too fast can cause a different and equally dangerous form of brain injury. Guidelines generally recommend lowering sodium by no more than 8 mmol/L in any 24-hour period for patients at risk of complications. This means frequent blood draws to monitor progress, and adjustments to the rate of fluid replacement throughout the day.
For people with diabetes insipidus, replacing the missing hormone with a synthetic version called desmopressin resolves the central form effectively. The nephrogenic form, where kidneys don’t respond to the hormone, requires a different approach, often focusing on removing the offending medication, increasing fluid intake, and sometimes using dietary changes to reduce how hard the kidneys need to work to concentrate urine.