Hyponatremia (Low Sodium): Symptoms, Causes, and Recovery

Hyponatremia simply means low sodium in the blood — a serum level below 135 mmol/L, where the normal range is 135–145. It is the most common electrolyte abnormality doctors see, turning up in roughly 1 in 5 hospitalized patients, and its symptoms can be confusingly vague: a dull headache, a wave of nausea, foggy thinking, deep fatigue, a wobbly unsteadiness, and in older adults a stumble or fall can all trace back to the same low number. Here is the key idea that makes sense of all of it: in most cases low sodium is not really a salt-shortage problem — it is a water problem. Sodium is the main mineral dissolved in the fluid outside your cells, and it is what holds water in the right place. When sodium gets diluted — usually because the body is holding on to too much water — water drifts into cells and they swell, and the most dangerous swelling happens in the brain, which is locked inside the rigid skull. That single fact explains why the symptoms are mostly neurological and why correcting low sodium has to be done at the right speed. The reassuring part: hyponatremia is found with a simple blood test, and once the cause is identified most cases are very treatable. This hub explains what hyponatremia is, why one diluted number causes so many different symptoms, what commonly causes it, and exactly how it is diagnosed and corrected — with deep-dive pages for each of the major symptoms.

Symptom Deep-Dive Pages

Table of Contents

1. Symptom Deep-Dive Pages
2. What Is Hyponatremia?
3. Why Low Sodium Causes So Many Different Symptoms
4. Common Causes of Low Sodium
5. Sodium, Water, and the Other Electrolytes
6. How Hyponatremia Is Diagnosed
7. How Low Sodium Is Corrected
8. When to Seek Care / Red Flags
9. Key Research Papers
10. Connections
11. Featured Videos

What Is Hyponatremia?

Sodium is an electrolyte — a mineral that carries an electrical charge when dissolved in body fluid. It is the main positively-charged particle in the fluid outside your cells (the blood and the fluid bathing your tissues), and its single most important job is to control where water goes. Water follows salt; wherever sodium is concentrated, water is drawn. Hyponatremia is the medical word for a blood (serum) sodium level below 135 mmol/L. A normal level sits between 135 and 145 mmol/L. The prefix “hypo-” means low and “-natremia” comes from natrium, the Latin name for sodium — which is also why sodium's chemical symbol is Na.

Here is the point that surprises most people, and it is worth stating plainly: hyponatremia is usually not caused by eating too little salt. A healthy person on a low-salt diet does not develop dangerous hyponatremia, because the kidneys are very good at conserving sodium when needed. Far more often, the sodium concentration falls because the body is holding on to too much water, which dilutes the sodium that is there — like adding water to soup until it tastes weak even though you never removed any salt. That is why doctors think of low sodium primarily as a problem of water balance, governed largely by a hormone called antidiuretic hormone (ADH, also called vasopressin) that tells the kidney how much water to keep.

How low the number falls, and especially how fast, matters a great deal, because the symptoms and the urgency scale with both. In plain terms:

• Mild (130–135 mmol/L) — Often there are no obvious symptoms, and the low value is found by chance on a routine blood test. But “mild” is misleading: even at this level, careful studies show subtle effects on attention, balance, and reaction time, which is why mild chronic hyponatremia in older adults is linked to unsteadiness and falls. People rarely connect a little extra tiredness or a vague off-balance feeling to their sodium.
• Moderate (125–129 mmol/L) — Now symptoms usually appear: headache, nausea, sluggish or foggy thinking, fatigue, and a sense of being unsteady on the feet. These are the symptoms of a brain that is beginning to swell. This is the range where people tend to feel unwell enough to seek help.
• Severe (below 125 mmol/L, especially if it falls quickly) — This can be a medical emergency. As brain swelling worsens, confusion deepens, and there is a real risk of seizures, a depressed level of consciousness, coma, and — in the most extreme, rapid cases — dangerous brain herniation. People with very low or rapidly falling sodium often need treatment in the hospital with close monitoring.

Speed is so important because the brain can protect itself against slow changes but not fast ones. When sodium falls gradually over many days, brain cells quietly pump out their own internal particles to match, which limits swelling — so a person can have a very low number (say, 118) yet feel only moderately unwell. When sodium falls quickly — over hours, as can happen with certain medications or extreme overdrinking — the brain has no time to adapt, and even a more modest drop can cause dramatic, dangerous symptoms. This is the central fact to carry through the rest of this page: with low sodium, the rate of change can matter as much as the number itself.

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Why Low Sodium Causes So Many Different Symptoms

The puzzle of hyponatremia is how a single diluted number can cause symptoms as different as a headache, a wave of nausea, foggy thinking, deep fatigue, and a fall. The answer is simpler than the long symptom list suggests, and it comes down to one idea: water follows sodium into cells, and the brain is where that swelling does the most harm.

Here is the mechanism in everyday language. Your cells sit in a bath of fluid, and sodium is the main dissolved particle in that surrounding bath. Normally the concentration of particles inside and outside a cell is balanced, so water has no reason to move in or out. When blood sodium falls, the bath outside the cells becomes dilute — it now has fewer particles per drop than the inside of the cell. Water always moves from the more watery side toward the more crowded side (this is osmosis), so water flows into the cells and they swell. Think of a raisin dropped into plain water plumping up as water seeps in.

Most tissues can tolerate a little swelling. The brain cannot, because it is sealed inside the rigid box of the skull with almost no room to expand. So as the brain's cells take on water, pressure builds, and the symptoms of hyponatremia are overwhelmingly the symptoms of a swelling, pressured brain — what doctors call cerebral edema. That single fact explains the whole symptom picture:

• Headache and nausea — among the earliest signs, both classic features of rising pressure inside the skull. Nausea is often the very first warning that sodium is falling. (Deep dives: Confusion & Headache and Nausea & Vomiting.)
• Foggy thinking, lethargy, and confusion — as swelling increases, the brain works less efficiently, producing the slowed, muddled, drowsy state that ranges from subtle inattention to outright confusion. See Brain Fog.
• Fatigue and unsteadiness — even mild hyponatremia subtly impairs balance, concentration, and reaction time, which feels like deep tiredness and a wobbly, off-balance sensation — and in older adults translates directly into falls. (Deep dive: Fatigue & Falls.)
• Muscle cramps and weakness — the fluid shifts and disturbed electrical environment around muscle cells contribute to cramping and weakness, particularly when sodium has been lost through heavy sweating or fluid losses. (Deep dive: Muscle Cramps.)
• Seizures, deep drowsiness, and coma — at the severe end, the most extreme expressions of a critically swollen brain, and the reason very low or rapidly falling sodium is a medical emergency.

This is the unifying theme to carry into the symptom pages: there is nothing mysterious about hyponatremia producing a scattershot of complaints. One diluted number drives water into brain cells, and a swelling brain — locked in the skull — is felt as headache, nausea, fog, unsteadiness, and, when severe, seizures.

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Common Causes of Low Sodium

Because hyponatremia is usually a problem of too much water relative to sodium, the causes sort into a few recognizable patterns. Doctors often begin by sizing up the person's body fluid: is there too little fluid overall (dehydration), about the right amount, or too much (visible swelling)? That single judgment narrows the cause quickly. Here are the ones worth knowing.

• The Syndrome of Inappropriate Antidiuretic Hormone (SIADH) — the single most common cause of hyponatremia in people who look neither dehydrated nor swollen. The body releases too much ADH, so the kidney holds on to water it should be releasing, diluting the sodium. SIADH has many triggers: many medications (especially certain antidepressants — the SSRIs — carbamazepine, and others), pneumonia and other lung diseases, brain injuries and infections, nausea and pain themselves, and some cancers. It is a leading reason an otherwise stable patient is found to have low sodium.
• Diuretics (“water pills”) — especially the thiazide type (such as hydrochlorothiazide and chlorthalidone), a very common and predictable cause. Thiazides impair the kidney's ability to dilute urine and excrete free water, so sodium drifts down. Older women on thiazides for blood pressure are a classic at-risk group, and this is one reason doctors check sodium after starting these drugs.
• Vomiting and diarrhea — lose fluid that contains salt; the body then releases ADH in response to the volume loss and holds on to water, which dilutes the remaining sodium. This is a frequent cause during a bad stomach illness, particularly when lost fluids are replaced mostly with plain water or low-salt drinks.
• Heart failure, cirrhosis, and kidney disease — the “too much fluid” group, where the body retains both salt and a relatively greater amount of water, leaving the person visibly swollen yet diluted. In heart failure and cirrhosis, a low sodium level is also an important marker of how advanced the underlying disease is.
• Drinking too much water (dilutional hyponatremia) — overwhelming the kidney's ability to excrete it. This happens in a few specific settings: endurance athletes who overdrink during a marathon (exercise-associated hyponatremia, made worse because exertion also raises ADH); a pattern called “beer potomania” and the “tea-and-toast” diet, where a very low-solute intake leaves the kidney unable to clear much water; psychogenic polydipsia (compulsive water drinking); and occasionally from certain recreational drugs that drive both thirst and ADH.
• Adrenal insufficiency (Addison's disease) and low thyroid — hormone deficiencies that impair the body's handling of sodium and water. Addison's disease, in which the adrenal glands fail to make enough cortisol and aldosterone, is an important, treatable, and often-missed cause of low sodium — classically paired with low blood pressure, fatigue, and sometimes high potassium.
• Salt-wasting and other kidney problems — less commonly, the kidney itself leaks sodium, as in certain salt-wasting states and after some brain injuries (cerebral salt wasting). These are part of why a specialist may be involved when the cause is not obvious.

One important distinction sits underneath all of this. A small number of hyponatremia cases are not a true water-balance problem at all but a laboratory artifact or a different kind of dilution: very high blood sugar (as in uncontrolled diabetes) pulls water out of cells and lowers the measured sodium, and in the past very high blood fats or proteins could fool older lab methods. A doctor keeps these in mind so that real and apparent hyponatremia are not confused.

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Sodium, Water, and the Other Electrolytes

If you remember one thing beyond the basics, make it this: low sodium is usually a story about water, and sodium never acts alone — it works in concert with water balance, potassium, and chloride. Understanding those partnerships clears up several common points of confusion.

Sodium and water are two sides of one coin. The body does not regulate the sodium amount directly; it regulates the sodium concentration, and it does this mostly by adjusting water. Thirst tells you to drink more water; the hormone ADH tells the kidney to hold water back. When that water-handling system goes wrong — too much ADH, or too much water taken in — sodium gets diluted even though the actual quantity of salt may be perfectly normal. This is why the treatment for many cases of low sodium is not “more salt” but “less water” (fluid restriction). For the broader picture of what sodium does in the body, see the Sodium overview.

Sodium and potassium are partners across the cell membrane. Sodium is concentrated outside cells; potassium is concentrated inside. A tiny molecular machine, the sodium-potassium pump, spends enormous energy keeping that separation, and it is the foundation of every nerve impulse and muscle contraction. The two minerals are also linked in blood pressure: diets high in sodium and low in potassium tend to raise blood pressure, while the reverse helps lower it — the basis of the potassium-rich, sodium-moderate eating patterns recommended for the heart. When potassium runs low at the same time as sodium (common with diuretics, vomiting, and diarrhea), both need attention — see Hypokalemia (Low Potassium).

Sodium and chloride travel together. The salt on your table is sodium chloride, and in the bloodstream sodium and chloride are the two electrolytes that move together most closely. Chloride is measured on the same routine blood panel and often shifts in parallel with sodium; the pattern between them helps doctors work out the underlying acid-base and fluid picture.

The practical takeaways:

• For most low-sodium problems, the lever to pull is water (and the hormone controlling it), not the salt shaker — which is why drinking less, not eating more salt, is so often the fix.
• When sodium is low, potassium is frequently low too (especially with diuretics or gut losses), and both are checked and corrected together.
• Sodium and chloride are read side by side on the same panel; together with potassium they give a fuller view of fluid and electrolyte balance than sodium alone.

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How Hyponatremia Is Diagnosed

The reassuring part of this story is that hyponatremia is usually easy to detect. It is most often found on a simple blood test — either a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP), both of which are routine, inexpensive, and report your serum sodium directly. Many people first learn their sodium is low not because they went looking for it, but because the value turned up on bloodwork ordered for something else, such as a check-up or a medication review. (For what the panel measures and how to read it, see the Comprehensive Metabolic Panel page.)

When the level is confirmed low, the goal shifts to two questions: how serious and how fast is it, and why is it happening. The “why” is genuinely detective work, because so many different conditions can lower sodium, and getting it right is what makes the treatment safe. Depending on the picture, a doctor may add:

• A clinical assessment of body fluid (volume status) — the doctor's bedside judgment of whether you are dry (dehydrated), normal, or fluid-overloaded (swollen ankles, raised neck veins). This single step splits the long list of causes into manageable groups and guides everything that follows.
• Serum and urine osmolality — osmolality measures how concentrated a fluid is. A low serum osmolality confirms the sodium is genuinely dilute (true hyponatremia) rather than a lab artifact. The urine osmolality then reveals whether the kidney is appropriately dumping the excess water (dilute urine, as in overdrinking) or inappropriately holding it (concentrated urine, as in SIADH or volume depletion).
• Urine sodium — a spot urine sodium helps separate the causes further: a low value points toward the body sensing low volume and clinging to salt (true volume depletion, heart failure, cirrhosis), while a higher value in a non-swollen person points toward SIADH, diuretics, or adrenal causes.
• Blood glucose and other checks — a glucose level rules out the artifact in which very high blood sugar lowers the measured sodium; thyroid and adrenal (cortisol) testing is added when the pattern suggests an underactive thyroid or Addison's disease.

One technical caveat worth knowing: the rate at which sodium has fallen is part of the diagnosis, not just the number. A person whose sodium dropped to 120 over a couple of days is in a very different situation from someone who has lived at 120 for months; this is why doctors hunt for any earlier sodium values and ask carefully about how quickly symptoms came on. That timeline directly shapes how fast it is safe to correct (next section).

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How Low Sodium Is Corrected

Treatment is matched to severity, symptoms, the cause, and — crucially — the speed at which the sodium fell. The unifying principles are: relieve a dangerously swollen brain quickly when needed, but raise the sodium back up slowly and in a controlled way, and treat the underlying cause so it does not simply happen again. There is a genuine art to the pacing here, and it is the reason significant hyponatremia is corrected under medical supervision rather than at home.

• Severe or symptomatic cases — controlled hospital treatment. When sodium is very low or someone has serious symptoms (seizures, severe confusion, a depressed level of consciousness), the swelling brain is the emergency, and doctors give a small, carefully measured dose of concentrated (hypertonic, 3%) saline to lift the sodium just enough to relieve the pressure. This is done with frequent blood checks in a monitored setting — never as a do-it-yourself measure.
• The cardinal rule — go slow on the way back up. Here is the counterintuitive danger: correcting low sodium too fast can cause its own devastating brain injury, called osmotic demyelination syndrome (once known as central pontine myelinolysis), in which the brain's protective coating is damaged days later. Because of this, guidelines cap how many units the sodium is allowed to rise per day. Raising sodium slowly is not caution for its own sake — it is how a serious, avoidable complication is prevented.
• The common “too much water” cases — fluid restriction. For SIADH and the fluid-overloaded states (heart failure, cirrhosis), the core treatment is often drinking less, not eating more salt — typically limiting total fluids to a set amount per day so the diluted sodium can concentrate back up. This surprises people who assume the answer must be salt tablets.
• The “too little fluid” cases — replace salt and fluid. When low sodium comes from true volume loss (vomiting, diarrhea, over-diuresis), giving balanced salt-containing fluid (such as intravenous normal saline, or oral rehydration) restores both volume and sodium and switches off the ADH signal that was holding water.
• Treat the cause. Replacing or restricting fluid without addressing why sodium dropped just resets the clock. That might mean stopping or changing a thiazide diuretic or a culprit medication, treating a pneumonia or other SIADH trigger, replacing thyroid or adrenal hormones, or managing the heart, liver, or kidney disease driving fluid retention.

For most people the outlook is good: once the cause is handled and sodium is brought back into range at a safe pace, the headache, nausea, fog, fatigue, and unsteadiness resolve. The two messages to take away are that significant low sodium belongs in medical hands, and that the speed of correction is, if anything, more important than the speed of the original fall.

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When to Seek Care / Red Flags

Most mildly low sodium causes vague symptoms rather than danger, and a non-urgent call to your doctor for a blood test is the right step for a persistent dull headache, ongoing nausea, unusual fatigue, or feeling off-balance — especially if you take a water pill (thiazide diuretic) or an antidepressant, have recently had a lot of vomiting or diarrhea, or have been drinking unusually large amounts of water. But certain symptoms mean sodium may be dangerously low and the brain could be at risk. Seek emergency care right away if you or someone else has any of the following:

• A seizure — any new convulsion is an emergency and is a recognized consequence of severe low sodium.
• Severe confusion, disorientation, or unusual behavior — especially if it is worsening, a sign of significant brain swelling.
• Excessive drowsiness, trouble waking, or loss of consciousness — a depressed level of alertness needs immediate attention.
• A severe, sudden, or rapidly worsening headache with vomiting — the combination suggests rising pressure inside the skull.
• Collapse or a fall with confusion — particularly in an older adult, where low sodium is a common and treatable contributor and a fall can cause serious injury.

People at higher risk — older adults (especially women) on thiazide diuretics, anyone on medications known to cause SIADH such as certain antidepressants, endurance athletes who drink heavily during events, and people with heart failure, cirrhosis, kidney disease, or known adrenal or thyroid problems — should have a lower threshold for getting checked, because in these settings sodium can drift down quietly. When in doubt, a quick blood test settles the question. For related symptoms, see Headache, Nausea & Vomiting, Dizziness, and Fatigue.

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Key Research Papers

1. Adrogué HJ, Madias NE (2000). Hyponatremia. New England Journal of Medicine;342(21):1581-1589. — DOI: 10.1056/NEJM200005253422107
2. Sterns RH (2015). Disorders of Plasma Sodium — Causes, Consequences, and Correction. New England Journal of Medicine;372(1):55-65. — DOI: 10.1056/NEJMra1404489
3. Spasovski G, Vanholder R, Allolio B, Annane D, Ball S, et al. (2014). Clinical practice guideline on diagnosis and treatment of hyponatraemia. Nephrology Dialysis Transplantation;29(suppl 2):i1-i39. — DOI: 10.1093/ndt/gfu040
4. Verbalis JG, Goldsmith SR, Greenberg A, Korzelius C, Schrier RW, et al. (2013). Diagnosis, Evaluation, and Treatment of Hyponatremia: Expert Panel Recommendations. The American Journal of Medicine;126(10 Suppl 1):S1-S42. — DOI: 10.1016/j.amjmed.2013.07.006
5. Ellison DH, Berl T (2007). The Syndrome of Inappropriate Antidiuresis. New England Journal of Medicine;356(20):2064-2072. — DOI: 10.1056/NEJMcp066837
6. Sterns RH (2018). Treatment of Severe Hyponatremia. Clinical Journal of the American Society of Nephrology;13(4):641-649. — DOI: 10.2215/CJN.10440917
7. Anderson RJ, Chung HM, Kluge R, Schrier RW (1985). Hyponatremia: A Prospective Analysis of Its Epidemiology and the Pathogenetic Role of Vasopressin. Annals of Internal Medicine;102(2):164-168. — DOI: 10.7326/0003-4819-102-2-164
8. Upadhyay A, Jaber BL, Madias NE (2006). Incidence and Prevalence of Hyponatremia. The American Journal of Medicine;119(7 Suppl 1):S30-S35. — DOI: 10.1016/j.amjmed.2006.05.005
9. Waikar SS, Mount DB, Curhan GC (2009). Mortality after Hospitalization with Mild, Moderate, and Severe Hyponatremia. The American Journal of Medicine;122(9):857-865. — DOI: 10.1016/j.amjmed.2009.01.027
10. Verbalis JG, Barsony J, Sugimura Y, Tian Y, Adams DJ, et al. (2010). Hyponatremia-Induced Osteoporosis. Journal of Bone and Mineral Research;25(3):554-563. — DOI: 10.1359/jbmr.090827
11. Siegel AJ, Verbalis JG, Clement S, Mendelson JH, Mello NK, et al. (2007). Hyponatremia in Marathon Runners due to Inappropriate Arginine Vasopressin Secretion. The American Journal of Medicine;120(5):461.e11-461.e17. — DOI: 10.1016/j.amjmed.2006.10.027
12. Dineen R, Thompson CJ, Sherlock M (2017). Hyponatraemia — presentations and management. Clinical Medicine;17(3):263-269. — DOI: 10.7861/clinmedicine.17-3-263

PubMed Topic Searches

• PubMed — Hyponatremia: causes, diagnosis, and management
• PubMed — SIADH and hyponatremia
• PubMed — Thiazide-induced hyponatremia
• PubMed — Hyponatremia, falls, and fractures in older adults
• PubMed — Osmotic demyelination and rapid correction of hyponatremia

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Connections

• Hyponatremia: Confusion & Headache
• Hyponatremia: Nausea & Vomiting
• Hyponatremia: Muscle Cramps
• Hyponatremia: Fatigue & Falls
• Sodium Overview
• Hypernatremia (High Sodium)
• Comprehensive Metabolic Panel
• Potassium
• Hypokalemia (Low Potassium)
• Chloride
• Magnesium
• Heart Failure
• Cirrhosis
• Kidney Disease
• Addison's Disease
• Headache
• Nausea & Vomiting
• Dizziness
• Fatigue
• Brain Fog

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