Low sodium in your blood means there’s too much water relative to sodium, diluting your body’s most important electrolyte below functional levels. The medical term is hyponatremia, and it’s the most common electrolyte disorder doctors encounter. Normal blood sodium falls between 135 and 145 milliequivalents per liter. Anything below 135 is considered low, and levels below 120 can become life-threatening.
Why Sodium Matters So Much
Sodium does far more than regulate thirst. It controls the balance of water moving in and out of every cell in your body, keeps your blood pressure stable, and allows your nerves and muscles to function. When sodium drops, water shifts into cells to try to equalize the concentration on both sides of cell membranes. This is manageable in most tissues, but your brain sits inside a rigid skull. Even modest swelling there can cause serious problems.
What Low Sodium Feels Like
Mild drops in sodium often feel vague and easy to dismiss. You might notice nausea, a dull headache, fatigue that doesn’t improve with rest, or muscle cramps. Many people chalk these symptoms up to dehydration or poor sleep, which is part of what makes low sodium tricky to catch early.
As levels fall further, symptoms become more distinct and more concerning. Confusion, irritability, and restlessness set in. Muscle weakness progresses beyond simple cramping. At dangerously low levels, the brain swelling becomes severe enough to trigger seizures or loss of consciousness. The speed of the drop matters as much as the number itself: sodium that falls quickly over hours is far more dangerous than a gradual decline over days or weeks, because the brain hasn’t had time to adapt.
The Most Common Causes
Low sodium is almost always a water problem, not a salt problem. Your body is holding onto too much water, losing too much sodium, or both. The underlying reason usually falls into one of a few categories.
Medications
Certain drugs are well-known culprits, especially in people over 65. Thiazide diuretics (water pills prescribed for blood pressure) are the most frequent offenders. Antidepressants, particularly SSRIs like citalopram, fluoxetine, paroxetine, and escitalopram, rank close behind. Antiseizure medications like carbamazepine, proton pump inhibitors used for acid reflux (omeprazole), and even some blood pressure medications and antibiotics like trimethoprim can push sodium down. If your sodium came back low and you recently started a new medication, that connection is worth raising with your doctor.
Hormonal and Organ Problems
A condition called SIADH is one of the most common medical causes. Normally, a hormone called vasopressin tells your kidneys when to hold onto water and when to let it go. In SIADH, that hormone stays elevated even when it shouldn’t, so your kidneys retain excess water and your blood becomes diluted. Drinking more water doesn’t fix it and can actually make it worse, because the excess fluid can’t be properly cleared.
SIADH itself has many triggers. Lung infections like pneumonia, head injuries, strokes, brain surgery, and certain cancers (small cell lung cancer is the most common) can all cause it. Some medications trigger it by amplifying vasopressin’s effects. Thyroid problems and adrenal gland insufficiency can also throw off this hormonal balance.
Heart failure and liver cirrhosis cause low sodium through a different mechanism. In these conditions, the body senses that blood isn’t circulating effectively, so it holds onto water to compensate. The result is the same: too much water diluting the sodium in your blood. Kidney disease, both acute and chronic, can also impair the kidneys’ ability to maintain the right sodium balance.
Drinking Too Much Water
This is the cause most people don’t expect. Overhydration, sometimes called water intoxication, happens when you take in more fluid than your kidneys can process. Your kidneys can typically handle about 0.8 to 1 liter per hour, so drinking well beyond that, especially without electrolytes, overwhelms the system.
Athletes in endurance events are particularly at risk. Marathon runners, ultramarathon participants, and long-distance cyclists who drink large volumes of plain water over several hours can develop exercise-associated hyponatremia. The key warning sign during an event is not losing weight or actually gaining weight, which signals fluid overload. Current guidance for athletes is straightforward: drink when you’re thirsty rather than forcing fluids on a schedule. Pre-loading with extra sodium before an event has been studied and doesn’t meaningfully help.
How Doctors Figure Out the Cause
A low sodium result on a basic blood panel is just the starting point. To find out why it’s low, doctors typically check the concentration of your urine. Very dilute urine (low osmolality) suggests you’re simply taking in too much water. More concentrated urine points toward a problem with how your body is handling water or sodium at the kidney level.
From there, the sodium content of your urine helps narrow things down further. Low urine sodium suggests your body is trying to conserve sodium, which happens with dehydration, vomiting, or diarrhea. High urine sodium suggests the kidneys are inappropriately dumping sodium, which points toward SIADH, kidney disease, or adrenal problems. Doctors also assess your overall fluid status: whether you seem dehydrated, normally hydrated, or swollen with excess fluid. Each pattern points to a different set of causes.
Why Correction Has to Be Slow
One of the most important things to understand about low sodium is that fixing it too fast can be as dangerous as the condition itself. When sodium has been low for more than a day or two, your brain cells adapt by releasing some of their internal solutes to prevent swelling. If sodium is then raised rapidly, water rushes out of those adapted brain cells too quickly, potentially damaging the protective coating around nerve fibers. This is called osmotic demyelination syndrome, and it can cause permanent neurological injury.
Current U.S. guidelines recommend raising sodium no faster than 10 to 12 millimoles per liter in a 24-hour period for most patients, and no faster than 8 millimoles per liter per day for people at higher risk (those with very low levels, chronic alcohol use, or severe malnutrition). European guidelines cap it at 10 per day. This is why significant hyponatremia is treated in a hospital with close monitoring rather than simply told to “eat more salt.”
Mild Low Sodium and What You Can Do
If your sodium is only slightly below normal and you feel fine, the cause is often identifiable and manageable. Reviewing your medications is the first step, since drug-induced hyponatremia frequently resolves once the offending medication is adjusted or switched. Reducing excessive water intake, if that’s a factor, can also bring levels back to normal.
For people with chronic conditions like heart failure or liver disease, low sodium tends to be an ongoing issue that’s managed alongside the underlying condition. Fluid restriction is a common strategy in these cases, limiting total daily fluid intake to help sodium levels stabilize.
If you’re an athlete or someone who exercises heavily, the practical takeaway is to match your fluid intake to your thirst rather than following rigid hydration schedules. Sports drinks containing electrolytes can help during prolonged activity lasting more than 60 to 90 minutes, but for shorter workouts, plain water in moderate amounts is fine. Monitoring your weight before and after long training sessions gives you a useful check: if you weigh the same or more afterward, you’re likely drinking more than you need.