Hyponatremia, a drop in blood sodium below the normal range, almost always traces back to the body holding on to more water than it should relative to its sodium stores. The causes are strikingly varied, from prescription medications and hormone disorders to marathon running and heavy beer drinking, but most of them converge on a single hormonal pathway. Understanding that shared thread, and the exceptions to it, makes the long list of triggers far easier to navigate.
The Hormone Behind Most Cases
Sodium concentration in blood is less about how much sodium you have and more about how much water surrounds it. The hormone that controls water retention is vasopressin, also called antidiuretic hormone (ADH). Vasopressin tells the kidneys to reabsorb water rather than excrete it, keeping the body’s fluid balance within a tight range. Without vasopressin, a person would urinate ten to fifteen liters a day, a condition known as diabetes insipidus.1Journal of Internal Medicine. Vasopressin: physiology, assessment and osmosensation Vasopressin is the main endocrine regulator of whole-body fluid balance, both at rest and during physical activity.2PubMed. Arginine vasopressin, fluid balance and exercise: is exercise-associated hyponatraemia a disorder of arginine vasopressin secretion?
When vasopressin is secreted at times or levels it shouldn’t be, the kidneys hold on to excess water, the blood becomes diluted, and sodium concentration falls. This “inappropriate” vasopressin release is the single most common mechanism underlying hyponatremia across a wide range of medical conditions.3PubMed. Hyponatremia and arginine vasopressin dysregulation: mechanisms, clinical consequences, and management The causes that follow are best understood as different reasons the body misjudges how much water it should keep.
SIADH and the Conditions That Trigger It
The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is the most frequently identified cause of euvolemic hyponatremia, meaning the person does not look obviously swollen or dehydrated. In SIADH, vasopressin is released even when the blood is already dilute enough that the hormone should be shut off. The kidneys obey the signal, retaining water and diluting sodium further.
SIADH has an enormous list of triggers. Malignant tumors can produce vasopressin on their own. Small-cell lung cancer is the classic example: tumor cells secrete ectopic ADH, and more severe hyponatremia in these patients predicts a worse prognosis.4PubMed Central. Syndrome of inappropriate secretion of anti-diuretic hormone (SIADH) as an initial presenting sign of non small cell lung cancer-case report and literature review But SIADH is not limited to cancer. It can also be caused by central nervous system disorders, lung infections, and positive-pressure ventilation, among other conditions.5PubMed. Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in malignant disease
Brain injuries deserve special mention. After a subarachnoid hemorrhage, hyponatremia is common and can result from either SIADH or a related but distinct condition called cerebral salt-wasting syndrome. Both produce similar lab findings, including low sodium, low blood osmolality, and concentrated urine, which makes telling them apart a genuine clinical challenge. The distinction matters because treatment is essentially opposite: SIADH calls for fluid restriction while cerebral salt wasting calls for salt replacement.6PubMed Central. Cerebral Salt-wasting Syndrome and Inappropriate Antidiuretic Hormone Syndrome after Subarachnoid Hemorrhaging
Medications That Lower Sodium
Drug-induced hyponatremia is extremely common, and two medication classes lead the pack: thiazide diuretics and antidepressants.
Thiazide diuretics, widely prescribed for high blood pressure, work by blocking sodium and chloride reabsorption in the kidney. This action interferes with the kidney’s ability to dilute urine, which in certain vulnerable people, particularly older adults and those with low body weight, can push sodium levels down.7PubMed Central. Thiazide-induced hyponatremia The risk is high enough that routine sodium monitoring is recommended after starting these drugs.
Antidepressants are the other major medication culprit. A meta-analysis found that exposure to antidepressants roughly tripled the odds of developing hyponatremia. The risk was not evenly spread across drug types: SNRIs had the highest event rates at around seven percent, followed by SSRIs at about six percent, then older tricyclics at roughly three percent. Mirtazapine and trazodone carried the lowest risk, under two percent.8PubMed Central. The risk of antidepressant-induced hyponatremia: A meta-analysis of antidepressant classes and compounds Another systematic review confirmed that both SSRIs and SNRIs more than doubled hyponatremia risk, and identified fluoxetine and venlafaxine as carrying the highest individual-drug risk within their respective classes.9PubMed Central. The risk of hyponatremia induced by SSRIs and SNRIs antidepressants: a systematic review and meta-analysis The mechanism appears to involve stimulation of vasopressin release, meaning antidepressant-induced hyponatremia is essentially drug-triggered SIADH.
Other medications linked to hyponatremia include anticonvulsants like carbamazepine, certain chemotherapy agents, and nonsteroidal anti-inflammatory drugs (NSAIDs). NSAIDs turn up repeatedly as a contributing factor in exercise-associated hyponatremia as well, likely because they impair the kidney’s ability to excrete free water.
Heart Failure and Liver Disease
When the heart or liver is failing, the body senses that its effective circulating blood volume is too low, even though total body water may actually be high. The response is to activate every sodium-and-water-retaining mechanism available, including the renin-angiotensin-aldosterone system, the sympathetic nervous system, and vasopressin. The result is fluid overload, edema, and dilutional hyponatremia. Clinicians call this hypervolemic hyponatremia because the patient has too much total body fluid.
In cirrhosis specifically, portal hypertension triggers widespread blood vessel dilation, which fools the body into thinking blood volume is low. As cirrhosis progresses, the kidneys become increasingly unable to excrete excess water. Non-osmotic secretion of vasopressin compounds the problem further, leading to more water retention and worsening hyponatremia over time.10PubMed Central. Hyponatremia in cirrhosis: pathophysiology and management Advanced kidney disease produces a similar picture through reduced ability to excrete dilute urine.
Hyponatremia in the setting of heart failure or cirrhosis is a marker of disease severity and carries a worse prognosis. Treating it effectively usually means addressing the underlying organ failure, not just the sodium number.
Endocrine Disorders
Several hormone deficiencies can cause hyponatremia through distinct but related pathways.
Adrenal insufficiency, whether from autoimmune destruction of the adrenal glands (Addison’s disease) or from pituitary problems, lowers both cortisol and aldosterone. Cortisol normally acts as a brake on vasopressin release. When cortisol drops, that brake is removed, and vasopressin levels climb, causing the kidneys to retain water. Low aldosterone compounds the problem by allowing the kidneys to waste sodium. The combination can produce dangerously low sodium levels.11PubMed Central. A Case of Severe Hyponatremia in A Patient With Primary Adrenal Insufficiency
Hypothyroidism is another endocrine cause, though the mechanism is more indirect. Severe hypothyroidism reduces cardiac output, which in turn triggers vasopressin release as the body tries to maintain blood pressure. The elevated vasopressin causes water retention and dilutional hyponatremia.12PubMed. Hypothyroidism-associated hyponatremia: mechanisms, implications and treatment Mild hypothyroidism rarely causes meaningful sodium drops; this is more of a concern with severe, untreated disease.
Exercise-Associated Hyponatremia
Marathon runners, ultraendurance athletes, and even military recruits can develop hyponatremia during or up to 24 hours after prolonged physical activity. Exercise-associated hyponatremia (EAH) has been reported in nearly every form of endurance exercise.13PubMed Central. EXERCISE-ASSOCIATED HYPONATREMIA
The biggest risk factor is straightforward: drinking more fluid than you lose. When someone sustains excess fluid intake that outpaces sweating, breathing, and urination for hours, sodium gets diluted.14PubMed Central. Exercise-Associated Hyponatremia in Marathon Runners But overdrinking alone does not fully explain EAH. Physical activity itself stimulates vasopressin secretion through non-osmotic pathways, meaning the kidneys hold on to water they would normally excrete at rest.15PubMed Central. Pathophysiology and treatment of exercise-associated hyponatremia Sodium lost in sweat plays a smaller role than people often assume.
Risk factors for EAH include exercising for more than four hours, high ambient temperatures, frequent fluid availability along the course, low body weight, limited endurance experience, and NSAID use.16PubMed Central. Exercise-Associated Hyponatremia in Marathon Runners The practical takeaway for athletes is to drink to thirst rather than following aggressive hydration schedules, which were common advice for decades but can push sodium dangerously low.
Dietary Patterns and Excessive Water Intake
You do not need to run a marathon to dilute your sodium through what you eat and drink. Two dietary patterns are particularly well known for causing hyponatremia through a mechanism that has nothing to do with vasopressin: low solute intake.
The kidneys need solutes, mainly sodium, potassium, and urea, to excrete water. When someone’s diet is very low in protein and electrolytes, the kidneys simply cannot produce enough dilute urine to get rid of excess water. “Beer potomania” is the colorful name for this phenomenon in heavy beer drinkers. Beer is extremely low in sodium, and chronic alcohol use suppresses protein breakdown, reducing the amount of urea available as a solute. The kidneys become unable to clear excess fluid, and dilutional hyponatremia results.17PubMed Central. “Beer Potomania” – A Syndrome of Severe Hyponatremia with Unique Pathophysiology: Case Studies and Literature Review
An analogous pattern in older adults goes by the name “tea and toast syndrome.” An elderly person living alone may subsist on tea, bread, and little else, producing an extremely low daily osmolar load. One case report documented a patient whose estimated dietary osmolar load was just 350 milliosmoles per day, well below what was needed for her kidneys to handle normal water intake.18PubMed Central. Tea and Toast Syndrome: A Case Report In these cases, the hyponatremia corrects once the diet improves.
On the extreme end, psychogenic polydipsia, compulsive water drinking driven by psychiatric illness, can overwhelm the kidneys entirely. In a study of twenty psychotic patients with polydipsia, water intake ranged from seven to forty-three liters daily, producing sodium levels as low as 98 milliequivalents per liter and symptoms including seizures, confusion, and coma.19JAMA Internal Medicine. Hyponatremia in Psychogenic Polydipsia Even in these patients, the vasopressin system played a role: many showed evidence of a “reset osmostat,” where vasopressin release kicked in at a lower-than-normal sodium threshold, sustaining the hyponatremia even when water intake was restricted.
Recreational Drugs and Hyponatremia
MDMA (ecstasy) is the recreational drug most closely linked to acute hyponatremia, and it creates a dangerous double hit. The drug stimulates vasopressin release, causing the kidneys to retain water, while simultaneously making users intensely thirsty, which leads to massive water intake. The combination can produce life-threatening water intoxication and SIADH within hours of a single dose.20PubMed Central. SIADH and water intoxication related to ecstasy Fatalities have been reported, particularly among young women at dance events where water is readily available and the hot environment encourages drinking. The advice to “stay hydrated” at raves, originally intended to prevent heat stroke, has in some cases contributed to fatal hyponatremia.
Hospital-Acquired Hyponatremia
Hospitals are, paradoxically, a common setting for new-onset hyponatremia. The reason often comes down to the fluids being infused. Hypotonic intravenous solutions, which contain more water relative to electrolytes than the blood does, can dilute sodium when given in volumes that exceed what the kidneys can clear.
This problem has been studied most extensively in children after surgery, a situation where vasopressin levels are already elevated from surgical stress, pain, and anesthesia. In a randomized trial comparing hypotonic and isotonic maintenance fluids after pediatric surgery, children receiving hypotonic fluids developed hyponatremia at nearly double the rate of those on isotonic fluids.21PubMed. Hypotonic versus isotonic maintenance fluids after surgery for children: a randomized controlled trial A major contributor is the traditional formula used to calculate children’s fluid requirements, which was designed for awake, healthy children and tends to overestimate water needs in a sedated, postoperative setting.22PubMed Central. Hyponatremia among Postoperative Children Administered with Hypotonic Fluids in a Tertiary Care Hospital: A Descriptive Cross-sectional Study This evidence has shifted many pediatric guidelines toward recommending isotonic maintenance fluids in hospitalized children.
Why Older Adults Are Especially Vulnerable
Hyponatremia is the most common electrolyte disorder in older adults, driven by a convergence of risk factors that accumulates with age.23PubMed. Special considerations of hyponatremia in the elderly patient Aging kidneys lose some of their ability to concentrate and dilute urine. Baseline vasopressin levels tend to be higher. Older adults are more likely to take thiazides and antidepressants, the two medication classes most associated with hyponatremia, and they are more likely to have heart failure, liver disease, or other chronic conditions that predispose them to low sodium.24PubMed Central. Hyponatremia in the elderly: challenges and solutions
Malnutrition in older adults adds another layer of risk through the low-solute mechanism discussed earlier. A significant proportion of elderly patients with hyponatremia have multiple overlapping causes rather than a single identifiable trigger, which complicates both diagnosis and treatment.25PubMed Central. Hyponatremia in the elderly: challenges and solutions
Classifying Causes by Fluid Status
Clinicians group hyponatremia by whether the patient’s total body fluid volume is low, normal, or high, because the grouping points toward different causes and different treatments.26PubMed Central. Hyponatremia: A practical approach
- Hypovolemic: The patient has lost both sodium and water, but proportionally more sodium. Causes include severe vomiting, diarrhea, heavy sweating, and certain kidney conditions. The body responds by releasing vasopressin to conserve water, which keeps the sodium diluted.
- Euvolemic: Total body water is slightly increased but the patient does not appear obviously volume-overloaded. SIADH is the dominant cause, along with hypothyroidism, adrenal insufficiency, and many medications.
- Hypervolemic: The patient has excess total body water and sodium, but water has accumulated faster than sodium. Heart failure, cirrhosis, and advanced kidney disease are the primary examples.
This framework is useful for diagnosis, but in practice many patients, particularly older adults on multiple medications, do not fit neatly into one category.
When the Lab Result Itself Is Wrong
Not every low sodium reading reflects true hyponatremia. Pseudohyponatremia is a laboratory artifact in which serum sodium reads falsely low because of unusually high levels of lipids or proteins in the blood. The sodium is actually fine; the measurement method is being fooled by the extra non-aqueous components in the sample. Direct ion-selective electrode measurement of sodium confirms normal levels in these cases.27PubMed Central. Pseudohyponatremia: A Potentially Dangerous Laboratory Artifact Pseudohyponatremia is rare, but it matters because treating a patient for low sodium they do not actually have can cause real harm.
There is also a category called translocational hyponatremia, seen most often in uncontrolled diabetes. Very high blood sugar pulls water out of cells and into the bloodstream, diluting sodium. The sodium level is genuinely low on the lab report, but the cause is the osmotic shift from glucose rather than a water-balance problem. When blood sugar is brought under control, sodium typically normalizes on its own.
A Rare Genetic Form
Most causes of hyponatremia are acquired, but there is at least one genetic form. Researchers described two infants who presented with SIADH-like symptoms, yet their vasopressin levels were undetectable. It turned out they had gain-of-function mutations in the gene for the V2 vasopressin receptor, causing the receptor to be permanently switched on even without the hormone present. This was termed “nephrogenic syndrome of inappropriate antidiuresis.”28PubMed Central. Nephrogenic syndrome of inappropriate antidiuresis The condition is exceedingly rare, but it illustrates that the problem can lie anywhere along the vasopressin signaling chain, from the hormone itself to the receptor it acts on.
The Danger of Correcting Too Quickly
One of the most important things about hyponatremia, and one that often surprises people, is that fixing it too fast can be worse than the condition itself. When sodium has been low for more than about 48 hours, brain cells adapt by shedding internal solutes to prevent swelling. If sodium is then corrected rapidly, water rushes out of those adapted brain cells, and the result can be osmotic demyelination syndrome, a devastating neurological injury.29PubMed Central. Osmotic Demyelination Syndrome following Correction of Hyponatremia by ≤10 mEq/L per Day
The early cellular damage appears to start with astrocytes, the brain’s support cells. Rapid correction triggers astrocyte death, which then disrupts their communication with the cells that produce myelin, the insulating sheath around nerve fibers. Inflammation follows, and eventually the myelin breaks down.30PubMed Central. Astrocytes are an early target in osmotic demyelination syndrome The syndrome can cause paralysis, difficulty speaking and swallowing, and in severe cases, a locked-in state. This is why guidelines set strict limits on how many milliequivalents per liter sodium should rise in a given time window, and why identifying and addressing the underlying cause, rather than just chasing the number, is the preferred approach.
How Aquaporins Fit In
The molecular machinery that actually moves water in and out of kidney cells consists of specialized channel proteins called aquaporins. At least seven types are expressed in the kidneys, and they play roles in both short-term and long-term water balance.31PubMed Central. Aquaporins: The renal water channels Vasopressin exerts its water-retaining effect largely by causing aquaporin-2 channels to be inserted into the kidney’s collecting duct cells, making those cells permeable to water. When vasopressin is absent, aquaporin-2 is pulled back inside the cells, the collecting duct becomes waterproof, and dilute urine flows out. Drugs being developed to treat hyponatremia, called vaptans, work by blocking the vasopressin receptor so that aquaporin-2 never gets the signal to open, allowing the kidneys to excrete water even when vasopressin levels are inappropriately high.