When sodium drops too low in your blood, water shifts into your cells and causes them to swell. This is especially dangerous in the brain, which has no room to expand inside the skull. A healthy blood sodium level falls between 135 and 145 millimoles per liter, and anything below 135 is considered low, a condition called hyponatremia. The effects range from barely noticeable fatigue to life-threatening seizures, depending on how far and how fast your levels fall.
Why Low Sodium Makes Cells Swell
Sodium controls how water moves between the inside and outside of your cells. When the concentration of sodium in your blood drops, water naturally flows toward the area with less sodium, which is now outside the cells. That means water rushes into your cells, inflating them like overfilled balloons.
In most of your body, a little extra swelling is tolerable. In your brain, it’s not. The skull is rigid, so swollen brain tissue presses against bone. This raises the pressure inside your head, reduces blood flow to brain tissue, and starves cells of oxygen. In severe cases, the brain can be pushed downward through the base of the skull, a process called herniation that can be fatal.
Your brain does try to protect itself. In the early stages, it pushes out cerebrospinal fluid and blood to make room. Then brain cells start dumping sodium, potassium, and amino acids to lower the water concentration inside themselves and slow the swelling. This adaptation works reasonably well when sodium drops gradually over days or weeks. When it plummets within hours, the brain simply can’t compensate fast enough.
Symptoms From Mild to Severe
When sodium falls slowly (chronic hyponatremia), the body partially adjusts, and symptoms tend to be subtle. You might feel unusually tired, have trouble concentrating, or notice your thinking feels foggy. These symptoms are easy to dismiss as stress or poor sleep, which is one reason low sodium often goes undetected for a while.
As levels drop further, more obvious symptoms appear: muscle cramps or weakness, nausea, vomiting, and headaches. Confusion and other mental changes follow. At the most dangerous end of the spectrum, acute hyponatremia, where blood sodium crashes rapidly, you can experience seizures, loss of consciousness, or coma. Untreated severe hyponatremia can cause permanent brain damage or death.
The speed of the drop matters as much as the number itself. Someone whose sodium drifts down to 125 over two weeks may feel mildly off. Someone whose sodium hits 125 in a matter of hours could be seizing. That distinction between chronic and acute hyponatremia shapes everything about how dangerous the situation is and how aggressively it needs to be treated.
The Most Common Causes
Low sodium isn’t a single disease. It’s the end result of many different conditions, and figuring out the underlying cause is key to fixing it. The causes generally fall into three categories based on how much fluid is in your body.
In the first category, you’ve lost both water and sodium, but you’ve lost more sodium than water. This happens with prolonged vomiting, severe diarrhea, or heavy sweating without adequate salt replacement. Dehydration from these causes pulls your sodium down.
In the second category, your total sodium is normal but you have too much water diluting it. The most common culprit here is a condition called SIADH (syndrome of inappropriate antidiuretic hormone). Your body produces too much of a hormone that tells your kidneys to hold onto water, so your blood becomes diluted. SIADH is especially common in people recovering from surgery, where the combination of IV fluids, pain medications, and the body’s stress response all drive excess water retention. It’s also frequently seen in people with lung diseases, particularly pneumonia and small-cell lung cancer.
In the third category, your body is retaining both sodium and water, but water more so. This pattern shows up in heart failure and liver cirrhosis, where fluid accumulates throughout the body and dilutes the sodium in your blood.
Medications That Lower Sodium
Two of the most widely prescribed classes of drugs are known to cause low sodium. Thiazide diuretics, commonly used for high blood pressure, interfere with the kidneys’ ability to concentrate urine properly and cause direct sodium loss. SSRIs, a common type of antidepressant, can trigger excess release of the water-retention hormone, effectively producing the same dilution effect as SIADH. When both medications are taken together, the risk of severe hyponatremia increases substantially.
Low Sodium From Overhydration During Exercise
Marathon runners, hikers, and other endurance athletes face a specific type of low sodium that has nothing to do with illness. During prolonged exercise, your kidneys slow urine production by 20 to 60 percent as blood flow shifts to working muscles. This is a normal response, but it means your body temporarily loses much of its ability to get rid of excess water. If you drink more fluid than you’re sweating out, your blood sodium drops, sometimes dangerously.
The clearest warning sign is gaining weight during a workout. If you weigh more after a long run than before, you drank too much. The practical prevention strategy is straightforward: don’t drink more than you sweat, and choose beverages or foods that contain sodium to replace what’s lost in sweat. Plain water in large volumes is actually riskier than a drink with some salt in it, because it dilutes your blood without replacing what you’ve lost.
How Low Sodium Is Diagnosed
A basic blood test reveals low sodium, but that’s only the starting point. To figure out why your sodium is low, doctors typically order a few additional tests. Urine sodium and urine concentration measurements help distinguish between the major categories. For example, very dilute urine suggests you’re simply drinking too much water, while concentrated urine with high sodium content points toward SIADH. Low urine sodium, on the other hand, suggests your body is trying to hold onto every bit of sodium it can, which is typical when the real problem is fluid loss or poor circulation from heart failure.
These patterns guide treatment in completely different directions, which is why identifying the root cause matters so much. Giving extra fluid to someone whose problem is already too much water would make things worse.
Why Correcting Sodium Too Fast Is Dangerous
One of the most counterintuitive things about low sodium is that fixing it too quickly can be just as harmful as the condition itself. When sodium has been low for more than a day or two, brain cells have already adapted by shedding internal solutes to prevent swelling. If blood sodium is then raised rapidly, water rushes out of those adapted brain cells, causing them to shrink. This can destroy the protective coating around nerve fibers in the brainstem, a condition called osmotic demyelination syndrome.
Research shows that raising sodium by more than 8 millimoles per liter in 24 hours is statistically associated with a higher risk of this complication. The damage can cause difficulty speaking, swallowing, or moving, and in severe cases it’s irreversible. This is why severe hyponatremia is managed in a hospital with frequent blood draws to monitor the correction rate carefully.
What Emergency Symptoms Look Like
Certain symptoms signal that low sodium has become a medical emergency. Persistent vomiting, seizures, and reduced consciousness or coma are classified as severe. Confusion, headache, and nausea without vomiting are considered moderately severe but still warrant urgent evaluation. In both cases, treatment with concentrated saline solution is started immediately, regardless of what’s causing the low sodium, because the priority shifts to preventing brain damage.
For people with chronic, mildly low sodium, treatment focuses on addressing the underlying cause: adjusting medications, managing heart failure, restricting fluid intake, or treating the infection or hormone imbalance that triggered it. The timeline for recovery depends entirely on what’s driving the problem. Medication-induced hyponatremia often resolves within days of stopping the drug, while sodium tied to a chronic illness like heart failure requires ongoing management.