Low sodium in the blood, known medically as hyponatremia, means your body has too much water relative to sodium. The formal cutoff is a serum sodium level below 135 mEq/L, with the normal range sitting between 135 and 145 mEq/L.1Cureus. Abnormal Presentation of Severe Hyponatremia It can stem from something as benign as drinking too much water during a marathon or as serious as heart failure or liver disease. What makes low sodium tricky is that the symptoms overlap with a dozen other conditions, mild cases often fly under the radar, and the treatment itself can be dangerous if done too fast.
Why Sodium Matters So Much
Sodium is the main positively charged particle floating in the fluid outside your cells. The volume of that fluid is directly tied to how much sodium your body holds.2Journal of Korean Society of Pediatric Nephrology. Water and Sodium Balance of Body Fluid Your body keeps sodium and water in a tight balance using a hormone called vasopressin (also called antidiuretic hormone, or ADH). When the concentration of particles in your blood rises even slightly, sensors in the brain trigger thirst and tell the kidneys to hold onto water. When it drops, ADH secretion falls and you urinate out the excess.3PubMed. Vasopressin: physiology, assessment and osmosensation The system is sensitive enough that healthy people keep their blood concentration within a very narrow range.4PubMed. Osmoregulation and control of vasopressin secretion in healthy humans
When sodium drops too low, the fluid outside your cells becomes more dilute than the fluid inside them. Water moves into cells to even things out, and cells swell. Most cells can tolerate a little of this, but the brain sits inside a rigid skull, so swelling there creates pressure fast. That pressure is where the dangerous symptoms come from.
What Low Sodium Feels Like
Mild hyponatremia, with sodium levels in the low 130s, often produces no obvious symptoms. You might feel a vague sense of being “off,” with mild nausea, a dull headache, or difficulty concentrating. Many people write it off as fatigue or dehydration.
When sodium drops below about 125 mEq/L, the situation becomes more serious. Symptoms at that level often include confusion, extreme drowsiness, seizures, and in the worst cases, coma, all driven by brain swelling.5PubMed. Hyponatremia: cerebral symptoms and role in central pontine myelinolysis However, the severity depends enormously on how fast the drop happened, not just how low the number goes. A person whose sodium drifted down to 118 over several weeks may walk around with nothing worse than mild confusion, while someone whose sodium plunged to 125 in a few hours could have a seizure.
Why Speed Matters More Than the Number
When sodium falls quickly, the brain has no time to protect itself, and swelling happens almost immediately.6PubMed Central. Hyponatremia and the Brain But the brain has a clever defense mechanism for gradual drops. Over hours and days, brain cells push out potassium, chloride, amino acids, and other small molecules to reduce their internal concentration, drawing water back out and shrinking toward their normal size.7PubMed. Mechanisms counteracting swelling in brain cells during hyponatremia The early phase of this adjustment relies mostly on ions like potassium, while the later phase uses organic molecules. By the time a chronic case has been going on for a few days, the brain has shed enough of these particles that it can tolerate a surprisingly low sodium level without dramatic symptoms.
This adaptation is a double-edged sword, though. Once brain cells have shed those molecules, they have essentially recalibrated to function at a lower sodium level. If a doctor then raises sodium back to normal too quickly, the newly “shrunk” brain cells lose water rapidly, and the resulting damage to nerve fibers can be devastating. That condition, called osmotic demyelination syndrome, involves destruction of the protective coating around nerve fibers in the brainstem and elsewhere. A sudden rise in sodium overwhelms the brain’s ability to re-absorb the molecules it dumped, leading to inflammation and permanent injury.8PubMed Central. Osmotic Demyelination Syndrome Following Rapid Correction of Hyponatremia in a Young Woman: A Case Report and Review of Literature Animal research has shown that brain damage occurs when sodium rises by more than about 20 to 25 mEq/L in a 24-hour period, and that re-lowering sodium after an overshoot can help prevent the damage.9PubMed. Prevention of brain demyelination in rats after excessive correction of chronic hyponatremia by serum sodium lowering In clinical practice, doctors typically aim to raise sodium by no more than about 8 to 10 mEq/L per day to stay safely below that threshold.
The Most Common Causes
Low sodium is not one disease. It is the downstream result of dozens of different problems, and figuring out which one is responsible changes the treatment completely. Doctors generally sort causes into three buckets based on your overall fluid status: too little total body fluid, too much total body fluid, or a normal amount of fluid with the wrong hormone signal.
Low Fluid Volume
When you lose both sodium and water, but sodium is lost faster, your blood sodium falls. This happens with severe vomiting, prolonged diarrhea, heavy sweating, or overuse of diuretic medications. Certain diuretics are among the most common drug culprits for hyponatremia, alongside antidepressants, anticonvulsants, and antipsychotics.10PubMed Central. Drug-Induced Hyponatremia: Insights into Pharmacological Mechanisms and Clinical Practice Management Thiazide diuretics, commonly prescribed for high blood pressure, are particularly notorious because they impair the kidney’s ability to dilute urine while also causing sodium to leave the body.
High Fluid Volume
In conditions like heart failure, liver cirrhosis, or severe kidney disease, the body retains enormous amounts of both sodium and water, but water is retained disproportionately. In cirrhosis, for instance, the liver disease causes blood vessels to dilate, which tricks the body into thinking blood volume is low. The kidneys respond by holding onto sodium and water aggressively, and ADH is released even though it shouldn’t be, piling on extra water and diluting sodium further.11PubMed Central. Hyponatremia in cirrhosis: pathophysiology and management The person looks swollen with fluid (edema, swollen ankles, sometimes a distended abdomen), yet their blood sodium is low. Paradoxically, the treatment in this scenario is often to restrict water, not to give salt.
Normal Fluid Volume with Inappropriate ADH
This is the category that confuses people the most. In the syndrome of inappropriate antidiuretic hormone secretion (SIADH), your body releases ADH when it shouldn’t. Instead of the hormone responding to genuine changes in blood concentration, it runs on autopilot. The kidneys dutifully hold onto extra water, diluting your sodium, while the body simultaneously activates mechanisms to dump sodium in the urine. The result is a stable but too-low sodium level in someone who looks clinically normal and is neither dehydrated nor swollen.12Australian Family Physician. The suspect – SIADH SIADH can be triggered by lung diseases, brain injuries, certain cancers, pain, nausea, and a long list of medications. It is one of the most frequent causes of low sodium seen in hospitals.
How Doctors Figure Out Which Type You Have
The diagnostic process moves through a logical sequence using a handful of lab values. The first step is measuring overall blood concentration (osmolality). If it is normal or high despite the low sodium number, the reading may be a lab artifact rather than real hyponatremia. If blood concentration is genuinely low, the next question is whether the kidneys are responding properly by making very dilute urine. A highly dilute urine tells the doctor that ADH is appropriately suppressed and the problem is probably just too much water intake. Concentrated urine means ADH is active when it shouldn’t be. Finally, the sodium concentration in the urine helps distinguish whether the body is holding onto sodium (suggesting low blood volume) or dumping it (pointing toward SIADH).13PubMed Central. The hyponatremic patient: a systematic approach to laboratory diagnosis
When the Number Is Fake
Sometimes the lab tells you sodium is low when it really isn’t. This happens most often in people with very high levels of fats (triglycerides or cholesterol) or proteins in the blood. The excess fat or protein physically displaces water in the sample, and the traditional lab technique measures sodium in the total sample volume rather than just the water portion. The result is a falsely low reading called pseudohyponatremia. Crucially, the actual concentration of sodium in the watery part of the blood is perfectly normal, which means the brain and other organs are not affected at all.14PubMed Central. Pseudohyponatremia: A Concise Guide to Diagnosis and Management in Clinical Practice Modern blood gas analyzers that measure sodium directly (ion-selective electrodes on undiluted samples) typically do not produce this artifact, but indirect methods still used in many lab panels can. If your sodium comes back low and you have no symptoms, your doctor may want to recheck it with a different method before starting any treatment.
Athletes and Overhydration
Exercise-associated hyponatremia is one of the more common ways otherwise healthy people end up with dangerously low sodium. It happens most frequently during prolonged endurance events like marathons, ultramarathons, and triathlons. For years, the intuitive explanation was that athletes lose too much salt in sweat, but research has shown the bigger problem is drinking too much water. Physical activity triggers ADH release even when the body does not need extra water, and if a person is simultaneously drinking more than their kidneys can excrete, sodium plummets.15PubMed Central. Pathophysiology and treatment of exercise-associated hyponatremia Sweat-related sodium loss plays a role, but it is not the main driver.
The practical takeaway for endurance athletes is to drink to thirst rather than on a fixed schedule. Sports drinks help to a degree because they contain sodium, but they are still much more dilute than blood, so guzzling large volumes of a sports drink can still cause problems. Smaller, slower athletes who take longer to finish a race are at higher risk because they have more time to overdrink. If you feel nauseous, confused, or develop a splitting headache after a long event, resist the urge to keep drinking water, as that will make things worse.
Medications That Catch People Off Guard
Drug-induced hyponatremia deserves its own emphasis because it is so common and so often overlooked. The usual suspects include thiazide diuretics, selective serotonin reuptake inhibitors (SSRIs) and other antidepressants, certain anti-seizure medications like carbamazepine, and some antipsychotic drugs.10PubMed Central. Drug-Induced Hyponatremia: Insights into Pharmacological Mechanisms and Clinical Practice Management Less obvious contributors include some chemotherapy agents, anti-inflammatory drugs, and even proton pump inhibitors used for acid reflux.
The risk tends to be highest in the first few weeks after starting a new medication, and it climbs with age. If you recently started a new drug and begin feeling unusually tired, mentally foggy, or nauseous, it is worth asking your doctor to check a basic metabolic panel. Many cases of drug-related low sodium resolve once the offending medication is switched or the dose is adjusted.
Low Sodium in Older Adults
Hyponatremia is strikingly common in people over 65. Aging kidneys become less efficient at concentrating urine, older adults tend to take more medications (each one adding a small risk), and the thirst mechanism dulls with age so people may not notice when their fluid balance is off. Even mild hyponatremia in older adults has been linked to gait problems, attention deficits, and an increased risk of falls and fractures.16PubMed Central. Hyponatremia independent of osteoporosis is associated with fracture occurrence
The relationship between low sodium and poor outcomes in elderly people is real, but the direction of causation is debated. A systematic review looking at hyponatremia and aging-related diseases concluded that low sodium may function more as a marker of unhealthy aging and declining overall health rather than a direct cause of falls or fractures. For cognitive decline, the evidence was even weaker for any causal relationship.17PubMed Central. Hyponatremia and aging-related diseases: key player or innocent bystander? A systematic review In other words, correcting an elderly person’s mild low sodium might not prevent them from falling, but finding low sodium should prompt a thorough look at what else is going wrong.
Drinking Too Much Water on Purpose
Outside the athletic context, excessive water drinking can also cause dangerous hyponatremia. This is sometimes seen in people with certain psychiatric conditions who compulsively drink enormous volumes of water, a pattern called psychogenic polydipsia. In a study of 20 psychiatric patients with this behavior, individuals drank anywhere from 7 to 43 liters of water per day, driving their sodium levels as low as 98 mEq/L and causing symptoms including headaches, seizures, confusion, and coma.18JAMA Internal Medicine. Hyponatremia in Psychogenic Polydipsia Even in these extreme cases, the kidneys tried to compensate by producing very dilute urine, but the sheer volume of incoming water overwhelmed them.19PubMed Central. Psychogenic polydipsia: the result, or cause of, deteriorating psychotic symptoms? A case report of the consequences of water intoxication
The wellness trend of aggressive daily water intake, while unlikely to push a healthy person with normal kidneys into serious trouble, can nudge sodium lower in people who already have risk factors. If you are on medications that affect sodium, have kidney or liver disease, or are elderly, the popular advice to “drink eight glasses a day” or carry a gallon jug everywhere may not be harmless.
Hospitalized Children and IV Fluids
Low sodium acquired in the hospital is a recognized risk in children receiving intravenous fluids. For decades, the standard practice was to give children dilute (hypotonic) IV solutions to maintain hydration, but this approach turned out to increase the risk of the sodium in their blood dropping to dangerous levels. The current recommendation is to use isotonic fluids for most children who need IV maintenance, avoiding solutions with less than 0.45% sodium chloride for routine hydration.20PubMed Central. Risk of acute hyponatremia in hospitalized children and youth receiving maintenance intravenous fluids This shift in practice was driven by case reports of previously healthy children developing brain swelling and sometimes fatal outcomes from hospital-acquired hyponatremia, a preventable complication.
Treatment Depends Entirely on the Cause
There is no single treatment for low sodium, and the wrong intervention can be worse than no treatment at all. In someone who is dehydrated and sodium-depleted, the answer is IV saline to replace what was lost. In SIADH or heart failure, giving saline could make things worse. Water restriction is a first-line approach for SIADH, and in severe or symptomatic cases, small boluses of concentrated (hypertonic) saline are given carefully to raise sodium just enough to stop brain swelling. Other tools include loop diuretics (which help the kidneys excrete water while sparing sodium), urea supplements, and a class of drugs called vaptans that block ADH’s action on the kidney.21PubMed. Hypertonic saline, isotonic saline, water restriction, long loops diuretics, urea or vaptans to treat hyponatremia
The overriding principle in every scenario is to raise sodium slowly and carefully. The risk of osmotic demyelination from overcorrection means doctors frequently recheck sodium levels every few hours during active treatment and will sometimes intentionally re-lower sodium if it climbs too fast.9PubMed. Prevention of brain demyelination in rats after excessive correction of chronic hyponatremia by serum sodium lowering
Why Your Body Craves Salt
Humans have a built-in appetite for sodium, and it works differently from hunger or thirst. Sodium appetite neurons in the brain are precisely timed to ingestive behavior, activating sharply when the body needs salt and shutting off almost immediately when sodium is detected in the mouth.22Cell. Neural Control of Ingestive Behavior Because the kidneys are extremely good at holding onto sodium under normal conditions, the body’s actual daily requirement is small, and the appetite circuit is designed for quick, precise corrections rather than sustained eating. This is why a salt craving often resolves after just a few bites of something salty.
People who are chronically sodium-depleted, whether from diuretic use, adrenal insufficiency, or heavy sweating over time, sometimes develop intense salt cravings that go beyond what most people experience. If you find yourself gravitating toward salty foods with unusual intensity, it may be worth having your electrolytes checked rather than assuming you just like chips.