Low sodium on a blood test means your blood sodium concentration has dropped below the normal range, a condition doctors call hyponatremia. The standard cutoff is 135 milliequivalents per liter (mEq/L), with normal falling between 135 and 145 mEq/L. It is the most common electrolyte abnormality found in clinical practice, and it ranges from a mild lab finding that barely registers as a problem to a medical emergency that can cause seizures or coma. What matters is not just the number itself but how fast it dropped, what caused it, and whether your body has had time to adjust.
Why Sodium Matters So Much
Sodium is the most plentiful positively charged particle in the fluid outside your cells, and it does more heavy lifting than most people realize. It helps maintain blood volume, keeps acid-base chemistry in check, preserves normal osmotic pressure, and supports the electrical signaling that cells depend on to function.1PubMed Central. Hyponatremia and lower normal serum sodium levels are associated with an increased risk of all‐cause death in heart failure patients Your kidneys are the main regulators. They use a network of hormonal signals, including the renin-angiotensin-aldosterone system, antidiuretic hormone (ADH), and natriuretic peptides, to decide how much sodium and water to hold onto or flush out at any given moment.2PubMed Central. Sodium Homeostasis, a Balance Necessary for Life
When sodium drops below the concentration inside your cells, water follows the osmotic gradient and flows into cells, making them swell. That swelling is tolerable in most tissues, but in the brain, which sits inside a rigid skull, it becomes dangerous fast.1PubMed Central. Hyponatremia and lower normal serum sodium levels are associated with an increased risk of all‐cause death in heart failure patients This is why the symptoms of low sodium are overwhelmingly neurological, and why the brain is the organ doctors worry about most.
Common Causes
Low sodium is rarely about not eating enough salt. In most cases, the problem is too much water relative to sodium, not too little sodium in absolute terms. Doctors sort causes by how much fluid is in your body, because that distinction changes the treatment entirely.3PubMed Central. Hyponatremia: A practical approach
- Too little fluid (hypovolemic): You have lost both sodium and water, but proportionally more sodium. This happens with severe vomiting, diarrhea, heavy sweating, or overuse of certain diuretics. Treatment typically involves replacing both fluid and sodium with normal saline.
- Normal fluid (euvolemic): Total body water is slightly expanded, but you do not look obviously swollen. The most common culprit here is the syndrome of inappropriate antidiuretic hormone secretion, or SIADH, where your body keeps producing ADH even when it should not, causing the kidneys to hold onto too much water and dilute the sodium.4PubMed Central. Syndrome of inappropriate antidiuretic hormone secretion: Revisiting a classical endocrine disorder Fluid restriction and, in some cases, concentrated saline are the mainstays of treatment.
- Too much fluid (hypervolemic): The body is retaining water and sodium, but water even more so, leading to diluted blood sodium. Heart failure is the most common reason for this type.5PubMed. Hypervolemic Hyponatremia in Heart Failure Liver cirrhosis causes a similar pattern because blood pools in the abdomen, tricking the kidneys into thinking blood volume is low and prompting them to retain more fluid.6PubMed. Sodium retention in heart failure and cirrhosis: potential role of natriuretic doses of mineralocorticoid antagonist?
SIADH deserves a closer look because it comes up so often. In a healthy body, ADH is released when blood becomes too concentrated, signaling the kidneys to reabsorb water and dilute things back to normal. In SIADH, that hormone keeps flowing regardless, and the kidneys obediently hold onto water the body does not need. The resulting dilutional hyponatremia triggers the body’s own volume-sensing systems to dump sodium in the urine, which only makes the problem worse.7Australian Family Physician. The suspect – SIADH Dozens of conditions can trigger SIADH, including lung diseases, brain injuries, cancers, and many common medications.
Medications That Lower Sodium
If you are older and your blood test shows low sodium, one of the first things your doctor should review is your medication list. The two drug classes most frequently implicated are thiazide diuretics (used for high blood pressure) and antidepressants, particularly SSRIs and SNRIs. These antidepressants can cause hyponatremia through the SIADH mechanism, prompting the body to retain water inappropriately.8PubMed Central. Hyponatremia Associated with the Use of Common Antidepressants in the All of Us Research Program A systematic review confirmed that while age, sex, kidney disease, and heart failure all modify the risk, antidepressant-associated hyponatremia is a real and independent effect, not just an artifact of other health problems.9PubMed Central. The risk of hyponatremia induced by SSRIs and SNRIs antidepressants: a systematic review and meta-analysis
The combination of a thiazide diuretic and an SSRI is especially risky. Case reports describe patients dropping to dangerously low levels, with one documented case reaching 104 mEq/L in a person taking both hydrochlorothiazide and an SSRI.10Semantic Scholar. Combination of Diuretics and SSRIs Leading Severe Hyponatremia: A Case Report If you take either class of drug and feel increasingly foggy, nauseated, or unsteady on your feet, it is worth asking your doctor to check your sodium.
What Symptoms to Expect
Mild hyponatremia, roughly in the 130–134 mEq/L range, often produces no obvious symptoms. You might feel slightly off, perhaps a little more tired or mildly nauseated, but nothing dramatic. As sodium falls further, the brain starts to struggle. Moderate drops bring headache, confusion, nausea, and lethargy. Severe hyponatremia, generally below 120 mEq/L, can cause seizures, loss of consciousness, respiratory arrest, and death.11PubMed Central. Effects of Hyponatremia on the Brain
Speed matters as much as depth. A sodium level of 125 mEq/L that developed over a week is far less dangerous than the same number reached in 24 hours. The brain has adaptive mechanisms: when sodium drops slowly, brain cells gradually push out organic solutes to reduce their own swelling. This adaptation buys time but has a downside, which becomes relevant during treatment.
Chronic hyponatremia, the kind that lingers at a mildly low level for weeks or months, was long considered harmless because patients did not look acutely ill. That view has changed. Research now links sustained low sodium to attention problems, unsteady gait, increased fall risk, and cognitive impairment. Studies in animal models show that prolonged mild hyponatremia impairs long-term memory formation and disrupts signaling at key brain junctions.12PubMed Central. Chronic Hyponatremia Causes Neurologic and Psychologic Impairments For elderly patients especially, the practical consequence is a higher risk of falls and hip fractures, driven by both the unsteady gait and a direct effect on bone: chronic low sodium appears to stimulate bone breakdown as the body tries to free up stored sodium, weakening the skeleton over time.13Oxford Academic (Nephrology Dialysis Transplantation). Is chronic hyponatremia a novel risk factor for hip fracture in the elderly?
How Doctors Investigate the Cause
A single low sodium value on a basic metabolic panel is just the starting point. Figuring out why it happened involves a stepwise process that leans heavily on a few additional lab tests. Blood osmolality is measured first to confirm that the low sodium is real and not an artifact. If blood osmolality is low (below roughly 280 mOsm/kg), the doctor then checks urine osmolality and urine sodium.14PubMed Central. The hyponatremic patient: a systematic approach to laboratory diagnosis
Urine osmolality below 100 mOsm/kg means the kidneys are doing their job correctly and diluting urine as much as possible; the problem is that you are simply taking in too much water. If urine osmolality is higher, something is impairing the kidneys’ ability to excrete water. From there, urine sodium helps narrow the field further. A urine sodium below 20 mmol/L suggests the body is volume-depleted and holding onto every bit of sodium it can, pointing toward dehydration-related causes. A urine sodium above 40 mmol/L in a patient who looks normally hydrated points toward SIADH.14PubMed Central. The hyponatremic patient: a systematic approach to laboratory diagnosis Thyroid function and cortisol levels are also typically checked, because both hypothyroidism and adrenal insufficiency can produce a picture that mimics SIADH.4PubMed Central. Syndrome of inappropriate antidiuretic hormone secretion: Revisiting a classical endocrine disorder
When the Number Is Wrong
Sometimes a blood test reports low sodium when the actual concentration in the blood’s water phase is perfectly normal. This is called pseudohyponatremia, and it is a lab artifact rather than a real medical problem. Traditional measurement methods, including indirect ion-selective electrodes and the older flame photometry technique, calculate sodium concentration based on a fixed assumption about how much of the blood sample is water. When blood contains abnormally high levels of fats or proteins, the water fraction shrinks, and these methods underestimate the sodium concentration.15The Journal for Nurse Practitioners. Pseudohyponatremia: A Concise Guide to Diagnosis and Management in Clinical Practice
Newer direct ion-selective electrode methods, which measure sodium in undiluted blood, tend to correct for this problem and give accurate readings even when lipids or proteins are elevated.16Clinical Chemistry. Determination of sodium with ion-selective electrodes If your sodium comes back low but you have no symptoms and your doctor knows you have very high cholesterol or a protein-producing condition like multiple myeloma, pseudohyponatremia should be on the list of explanations before anyone starts aggressive treatment.
Why Correction Speed Is Critical
Treating low sodium sounds simple on paper: give saline or restrict fluids until the number comes up. In practice, the speed of correction is one of the more dangerous decisions in hospital medicine. If sodium has been low for more than a day or two, the brain has already adapted by shedding solutes to reduce swelling. Raising sodium too quickly in that setting reverses the osmotic gradient abruptly, pulling water out of brain cells and damaging the myelin sheath that insulates nerve fibers. The result is osmotic demyelination syndrome, a devastating condition that can cause paralysis, difficulty speaking and swallowing, and locked-in syndrome.17PubMed Central. Osmotic Demyelination Syndrome Following Rapid Correction of Hyponatremia in a Young Woman: A Case Report and Review of Literature
The risk is not small. Among patients admitted with severe hyponatremia, rapid correction has been documented in roughly four out of ten cases, and nearly all patients who developed osmotic demyelination had experienced at least one episode of overcorrection.18PubMed Central. Risk Factors and Outcomes of Rapid Correction of Severe Hyponatremia Current guidelines generally advise raising sodium no more than about 8–10 mEq/L in any 24-hour period for chronic cases. Acute hyponatremia that developed within hours, such as in a marathon runner who over-hydrated, can be corrected more aggressively because the brain has not yet adapted.
Exercise-Associated Hyponatremia
Endurance athletes, particularly marathon runners, are an unexpected population at risk. Exercise-associated hyponatremia develops when someone drinks more fluid during prolonged exercise than their kidneys can excrete, especially if ADH remains elevated from the physical stress. The excess water dilutes blood sodium. A landmark study of Boston Marathon runners found that hyponatremia was strongly associated with drinking more than three liters during the race, racing for more than four hours, and gaining weight during the event. On statistical analysis, a race time above four hours carried roughly seven times the odds of developing hyponatremia compared with finishing in under three and a half hours.19PubMed. Hyponatremia among runners in the Boston Marathon
The mechanism is straightforward: during extended exercise, ADH production does not fully shut off, the kidneys cannot dump water fast enough, and the runner keeps drinking.20PubMed Central. Exercise-Associated Hyponatremia in Marathon Runners The practical takeaway for endurance athletes is to drink to thirst rather than forcing fluids on a schedule, and to be aware that sports drinks alone do not prevent the problem because most contain far less sodium than blood does.
Low Sodium in Older Adults
Older adults develop hyponatremia more often than any other age group, and the reasons stack up. Aging kidneys become less efficient at concentrating urine, baseline ADH levels tend to run higher, and the medication list grows longer. Thiazides and antidepressants are the most commonly implicated drugs, but the cause is frequently multifactorial. Endocrine issues, poor dietary intake (sometimes called “tea and toast” syndrome, where an elderly person eats so little protein and salt that the kidneys lack enough solute to excrete free water), and SIADH often overlap in the same patient.21PubMed Central. Hyponatremia in the elderly: challenges and solutions
The consequences in this population are especially harsh. Even mildly low sodium nudges up the risk of falls and fractures, as discussed above, and the cognitive effects can be mistaken for dementia or written off as normal aging. Monitoring sodium levels periodically is reasonable for any older person taking diuretics or antidepressants, even when they feel fine.
Children in the Hospital
Hyponatremia in hospitalized children is most commonly caused by the fluids they receive through an IV, not by the illness that brought them in. For decades, pediatric patients were routinely given hypotonic (low-sodium) IV fluids based on historical formulas that overestimated children’s free water needs. Multiple meta-analyses have since demonstrated that this practice substantially increases the risk of hyponatremia. One analysis found that children receiving hypotonic fluids had more than twice the risk of developing low sodium compared with those given isotonic fluids, and a more than fivefold increased risk of severe hyponatremia.22PubMed. Isotonic versus hypotonic maintenance IV fluids in hospitalized children: a meta-analysis Updated evidence with more trials confirms this pattern, showing that isotonic fluids cut the risk of mild hyponatremia by more than half at both early and later time points.23PubMed Central. Efficacy and safety of isotonic versus hypotonic intravenous maintenance fluids in hospitalized children: an updated systematic review and meta-analysis of randomized controlled trials Most pediatric guidelines now recommend isotonic maintenance fluids as the default, though the shift in practice has been gradual.
Beer Potomania and Low-Solute Diets
One of the more unusual causes of hyponatremia shows up in people who drink large volumes of beer with very little food. Beer is mostly water with minimal salt and protein. The kidneys need dissolved solutes in the urine to excrete free water efficiently; when solute delivery to the kidneys drops because the diet is almost entirely beer, the kidneys lose their capacity to get rid of excess water. The result is dilutional hyponatremia even without any abnormality in ADH.24PubMed Central. “Beer Potomania” – A Syndrome of Severe Hyponatremia with Unique Pathophysiology: Case Studies and Literature Review Alcohol itself adds to the problem by suppressing protein breakdown, further reducing the solute available for the kidneys to work with.25PubMed Central. Beer Potomania: A View on the Dynamic Process of Developing Hyponatremia
This mechanism is not exclusive to beer drinkers. Anyone on an extremely low-protein, low-salt diet who also drinks large amounts of fluid can develop the same problem. Crash dieters, elderly people eating very little, and people with eating disorders occasionally show up with hyponatremia driven by this same low-solute pathway. It is one more reason that the reflexive advice to “drink more water” is not always harmless.
The Relationship Between Low-Salt Diets and Blood Sodium
A common misconception is that eating less salt causes low blood sodium. For most people, it does not. Dietary sodium restriction is about reducing the total amount of sodium in the body to lower blood pressure, not about changing blood sodium concentration. The kidneys are remarkably good at adjusting excretion to maintain concentration within its tight range. You could cut your salt intake dramatically and your blood sodium would stay normal because the kidneys would simply excrete less sodium and retain less water to match.
That said, the relationship is not entirely irrelevant. Some researchers have pointed out that hyponatremia has become more common over roughly the same period that low-salt dietary guidelines became widespread, particularly in vulnerable populations like the elderly.26PubMed Central. The Hyponatremia Epidemic: A Frontier Too Far? Whether this is coincidence, confounding by the aging population and increased medication use, or a genuine contribution from salt restriction in already-vulnerable kidneys remains debated. For healthy adults, cutting back on salt is not going to drop your blood sodium. For frail elderly patients already on diuretics and drinking plenty of fluid, the margin of safety is thinner.
What Your Doctor Does with a Mildly Low Number
If your basic metabolic panel comes back with a sodium of, say, 133 mEq/L and you feel fine, the response is usually to recheck it, review your medications, and look at your fluid intake. A single mildly low reading in someone who is otherwise healthy and not on risky medications might be repeated in a few days without any intervention. If it persists, the stepwise workup of blood osmolality, urine osmolality, and urine sodium comes into play.
For the reader who just got a blood test result and is trying to make sense of a flagged sodium value: mild hyponatremia in isolation does not usually warrant a trip to the emergency room. It does warrant a conversation with your doctor, a review of what you are taking and drinking, and a plan to recheck. If you are experiencing confusion, persistent nausea, headaches, or unsteadiness, those symptoms paired with a low sodium reading move the situation from “monitor” to “evaluate urgently.” The number on the lab report tells the doctor what to investigate. It almost never tells the whole story by itself.