Low sodium, clinically called hyponatremia, means your blood sodium has dropped below 135 mEq/L (the normal range is 135 to 145 mEq/L). It’s one of the most common electrolyte abnormalities found on routine blood work, and the causes range from something as simple as drinking too much water to underlying conditions affecting your heart, liver, or hormones. Understanding why it happens starts with one key concept: low sodium is almost always about too much water relative to sodium, not too little sodium on its own.
How Your Body Regulates Sodium
Your brain produces a hormone called antidiuretic hormone (ADH) that tells your kidneys how much water to hold onto or release. When sodium levels start to dip, your body should suppress ADH so your kidneys flush out extra water and bring the ratio back into balance. When that system malfunctions, or when something overwhelms it, water accumulates in your bloodstream and dilutes sodium below normal levels.
Doctors classify low sodium into three broad categories based on your overall fluid status. In the first, you’ve lost both water and sodium (from vomiting, diarrhea, or heavy sweating), but you’ve lost proportionally more sodium than water. In the second, your total sodium is roughly normal but your body is holding onto excess water, diluting what’s there. In the third, both water and sodium are elevated, but water has increased far more, which happens in conditions like heart failure and liver disease. Each pattern points toward a different set of causes.
Medications That Lower Sodium
Medications are among the most common and most overlooked causes of low sodium. Two drug classes stand out: water pills (thiazide diuretics) and antidepressants, particularly SSRIs like sertraline, fluoxetine, and citalopram. Thiazide diuretics force your kidneys to excrete sodium directly, sometimes pushing levels below the safe range. When SSRIs and thiazide diuretics are taken together, the risk climbs sharply, with some studies finding the combination increases the odds of low sodium by 11 to 13 times compared to taking neither.
Among antidepressants, SSRIs and venlafaxine (an SNRI) carry the highest risk, with incidence rates reported as high as 40% for SSRIs and 70% for venlafaxine in some study populations. Older tricyclic antidepressants and mirtazapine carry a lower but still real risk. These medications can trigger the brain to release ADH inappropriately, causing the kidneys to retain water even when sodium is already dropping. The effect tends to appear within the first few weeks of starting or increasing a dose, and older adults are especially vulnerable.
Drinking Too Much Water
Healthy kidneys can clear roughly one liter of fluid per hour, or about 20 liters per day. That’s a generous buffer, but it’s not unlimited. Drinking water faster than your kidneys can process it dilutes blood sodium, sometimes dangerously. This happens most often in two situations: endurance exercise and compulsive water drinking.
Exercise-associated hyponatremia has killed marathon runners, football players, soldiers, and recreational athletes. The mechanism is straightforward: during prolonged physical activity, people drink large volumes of water or sports drinks that outpace their sweat and urine losses. At the same time, exercise triggers the release of ADH through pain, nausea, heat exposure, and physical stress, which slows the kidneys’ ability to shed the excess. The result is a rapid drop in sodium that can cause brain swelling. Deaths have been confirmed across a surprisingly wide range of activities, from ultramarathons to fraternity calisthenics to a 19-kilometer bike ride.
Outside of exercise, some people with certain psychiatric conditions drink compulsively large amounts of water throughout the day. Stress and illness can also reduce the kidneys’ hourly processing capacity well below one liter, lowering the threshold at which water intake becomes a problem.
Heart, Liver, and Kidney Disease
Chronic organ disease is a major driver of low sodium, especially heart failure. In heart failure, the heart pumps blood less effectively, which reduces pressure in the blood vessels. Your body interprets this as dehydration, even though you may actually have excess fluid (visible as swelling in your legs or abdomen). In response, it ramps up ADH, stimulates thirst, and instructs the kidneys to hold onto both water and sodium. But the water retention outpaces the sodium retention, so blood sodium falls.
Liver cirrhosis works through a similar mechanism. The damaged liver causes fluid to pool in the abdomen and blood vessels to dilate, which again tricks the body into sensing low volume and retaining water. Kidney disease directly impairs the organ’s ability to balance water and sodium excretion. In all three conditions, low sodium tends to develop gradually and can become a persistent, recurring problem rather than a one-time event.
Hormonal Causes
The most well-known hormonal cause is a condition called SIADH, which stands for syndrome of inappropriate antidiuretic hormone secretion. Normally, ADH rises when you’re dehydrated and falls when you have plenty of fluid. In SIADH, the brain keeps producing ADH regardless of how much water is already in your bloodstream. Your kidneys respond by concentrating your urine and holding onto water, which dilutes your blood sodium.
SIADH can be triggered by lung infections (especially pneumonia), brain injuries, cancers (particularly lung cancer), surgery, and certain medications. It’s the most common cause of low sodium in hospitalized patients. Adrenal insufficiency and severe hypothyroidism can also lower sodium, because both conditions impair the kidneys’ ability to excrete free water efficiently.
Vomiting, Diarrhea, and Heavy Sweating
Prolonged vomiting or diarrhea causes you to lose both water and sodium, but sodium losses often dominate. If you replace those losses by drinking plain water or other low-sodium fluids, you worsen the imbalance. This is one of the most common causes of low sodium in otherwise healthy people, and it’s why oral rehydration solutions contain salt, not just sugar and water. Heavy sweating during manual labor or heat exposure works the same way, especially if you’re replacing fluids with water alone over many hours.
What Low Sodium Feels Like
Mild drops in sodium often produce vague symptoms that are easy to attribute to something else: fatigue, difficulty concentrating, mild nausea, or a general feeling of being “off.” Chronic low sodium that develops slowly over weeks may only show up as persistent tiredness or subtle mental fogginess, because the brain has time to partially adapt.
When sodium falls quickly or drops below roughly 120 mEq/L, the symptoms become more serious. Headaches intensify. Confusion sets in. Muscle cramps and weakness develop. Nausea can progress to vomiting. At the most severe levels, the brain swells inside the skull, which can cause seizures, loss of consciousness, and, if untreated, permanent brain damage or death. Confusion, seizures, or difficulty waking someone up are emergencies.
How Low Sodium Is Diagnosed
A basic blood test (metabolic panel) reveals low sodium, but finding the number is only the first step. Identifying the cause requires additional tests. Doctors typically check urine sodium concentration and urine osmolality (a measure of how concentrated your urine is) to narrow down where the problem originates. Concentrated urine with high sodium suggests the kidneys are being told to hold water by ADH. Dilute urine with low sodium suggests you’re simply taking in more water than your body needs.
Your doctor will also assess your fluid status through a physical exam, looking for signs of dehydration (dry mouth, low blood pressure when standing) or fluid overload (swelling, weight gain). Combined with your medication list and medical history, these clues usually point to a cause.
How Low Sodium Is Corrected
Treatment depends entirely on the cause and how quickly sodium dropped. For mild, chronic cases, the fix may be as simple as restricting fluid intake, adjusting a medication, or treating the underlying condition. If a diuretic or antidepressant is responsible, switching to an alternative often resolves the problem within days to weeks.
For severe or rapidly developing cases, sodium is corrected with intravenous fluids in a hospital. The correction has to be slow and carefully monitored. Raising sodium too quickly, more than about 8 mEq/L in 24 hours, risks a rare but devastating complication called osmotic demyelination syndrome, which damages the protective coating on nerve cells in the brain. Interestingly, recent research found that correcting too slowly (less than 6 mEq/L over 24 hours) in patients with very low levels below 120 mEq/L was associated with higher mortality and longer hospital stays. This narrow window is why severe cases require intensive monitoring.
For exercise-related low sodium, prevention is the best approach. Drinking to thirst rather than on a fixed schedule, and choosing fluids with electrolytes during prolonged activity, significantly reduces the risk.