Serum sodium, measured as part of a routine blood panel, reflects the balance between the water and sodium in your bloodstream rather than simply how much salt you eat. The normal range sits at roughly 135 to 145 mmol/L. A reading below 135 is called hyponatremia (low sodium), and one above 145 is called hypernatremia (high sodium). Both directions carry real health consequences, but the causes, symptoms, and urgency differ considerably between them, and the story behind an abnormal number is almost always about water balance rather than salt intake.
How Your Body Keeps Sodium in Range
Sodium concentration in the blood is tightly regulated by a hormone called vasopressin (also known as antidiuretic hormone, or ADH). When the concentration of dissolved particles in your blood rises even slightly, the brain signals the release of vasopressin, which tells the kidneys to hold onto water. That retained water dilutes the blood back toward normal. When concentration drops, vasopressin secretion falls, and the kidneys let more water pass into urine, concentrating the blood again.1PubMed. Vasopressin: physiology, assessment and osmosensation
What many people don’t realize is that vasopressin also promotes sodium reabsorption deeper in the kidney’s filtration system. So the hormone is pulling a double shift: holding onto water and reclaiming sodium at the same time to maintain the concentration gradient the kidneys need to do their job.2Kidney International. Vasopressin regulation of renal sodium excretion This is why most sodium disorders trace back to problems with water handling, hormone signaling, or kidney function rather than to how much salt is on your dinner plate.
What Low Sodium Means
Hyponatremia is the more common direction of abnormality. Roughly 30% of hospitalized patients have low serum sodium at some point during their stay.3PubMed Central. Syndrome of inappropriate antidiuretic hormone secretion: Revisiting a classical endocrine disorder Despite the name suggesting a sodium problem, the issue in most cases is too much water relative to sodium, not too little sodium in the body.
One of the most frequent causes in a hospital setting is the syndrome of inappropriate antidiuresis (SIAD, sometimes called SIADH). In this condition, the body keeps producing vasopressin even when it shouldn’t, so the kidneys retain excess water and dilute the sodium. Underlying triggers for SIAD include lung disease, certain cancers, and neurological conditions.4Endocrine Reviews. Syndrome of Inappropriate Antidiuresis: From Pathophysiology to Management The hyponatremia in SIAD is dilutional: total body sodium may be normal or even slightly elevated, but the extra water pulls the concentration down.
Other common causes include:
- Thiazide diuretics: These blood-pressure medications are a well-known culprit. Although the traditional explanation blamed pure sodium loss, the picture is more complex. Thiazides stimulate vasopressin secretion, reduce the kidney’s ability to excrete free water, and may increase thirst, all of which contribute to dilutional low sodium.5PubMed Central. Thiazide-induced hyponatremia
- High blood sugar: In uncontrolled diabetes, elevated glucose in the blood pulls water out of cells and into the bloodstream, diluting sodium. Patients with extreme hyperglycemia are typically mildly hyponatremic for this reason.6PubMed Central. Corrected sodium levels for hyperglycemia is a better predictor than measured sodium levels for clinical outcomes among patients with extreme hyperglycemia
- Antidepressants: SSRIs and other antidepressants can lower serum sodium, probably through an SIAD-like mechanism involving excess vasopressin activity.7PubMed Central. Antidepressants and the risk of hyponatremia: a Danish register-based population study
- Vomiting, diarrhea, and heavy sweating: These cause actual sodium loss. When people replace the lost fluid with plain water or low-sodium beverages, the concentration drops further.
How Low Sodium Feels and Why It Matters
Mild hyponatremia, with sodium in the low 130s, often goes unnoticed or produces only subtle symptoms: slight nausea, headache, or a vague sense of being “off.” The trouble is that even mild, chronic low sodium can quietly cause harm. Research has linked it to unsteady gait, mild cognitive impairment, and a higher risk of falls and fractures in older adults. Part of the explanation is neurological, with impaired attention and balance, but there’s also a direct bone effect: the body appears to break down bone tissue to mobilize sodium stores, contributing to osteoporosis over time.8PubMed Central. Is chronic hyponatremia a novel risk factor for hip fracture in the elderly?
Severe or rapidly developing hyponatremia is a different beast entirely. When sodium drops fast, cells absorb excess water and swell. The brain is especially vulnerable because it sits inside the skull with no room to expand. Brain swelling from acute hyponatremia can cause seizures, loss of consciousness, and, in the worst cases, herniation and death.9PubMed Central. Hyponatremia and the Brain This is why emergency-room physicians pay close attention not just to how low the sodium is but to how quickly it got there.
The Correction Paradox
Treating severe hyponatremia comes with its own danger, and this is one of the more counterintuitive aspects of sodium management. If the sodium climbed too slowly over days or weeks, brain cells had time to adapt by shedding internal solutes to reduce swelling. They reached a fragile new equilibrium. Correct the sodium too fast, and you rip those adapted cells in the opposite direction: they shrink, and the insulating myelin around nerve fibers can break down, a condition called osmotic demyelination syndrome.
Landmark work in the 1980s established that raising serum sodium by more than 12 mmol/L per day in patients with severe chronic hyponatremia was associated with every case of neurologic complications in one hospital series, while patients corrected more slowly recovered uneventfully.10PubMed. Osmotic demyelination syndrome following correction of hyponatremia Animal research later showed that even after an overly rapid correction, lowering sodium back down within about 12 hours could prevent the demyelination from becoming permanent, as long as the total correction stayed within safe limits.11Kidney International. Prevention of brain demyelination in rats after excessive correction of chronic hyponatremia by serum sodium lowering In practice, hospital teams now monitor sodium levels frequently during correction and sometimes deliberately re-lower it if the number rises too fast.
What High Sodium Means
Hypernatremia, with serum sodium above 145 mmol/L, is almost always a story about not enough water rather than too much salt. The most common scenario is straightforward: a person who can’t drink enough, whether because they’re very old, very young, sedated, or cognitively impaired. Aging brings a diminished sense of thirst along with hormonal and kidney changes that make it harder to conserve water, which is why older adults in care facilities are particularly vulnerable to dehydration-driven hypernatremia.12PubMed. Clinical aspects of changes in water and sodium homeostasis in the elderly
A less common but instructive cause is diabetes insipidus, which has nothing to do with blood sugar. In this condition, either the brain fails to produce enough vasopressin (central diabetes insipidus) or the kidneys stop responding to it (nephrogenic diabetes insipidus). Either way, the kidneys dump enormous volumes of very dilute urine, and if fluid intake doesn’t keep pace, sodium concentration climbs.13PubMed Central. Diabetes Insipidus: Pathogenesis, Diagnosis, and Clinical Management
The symptoms of hypernatremia tend to reflect dehydration and neurological irritability: intense thirst (if the person can sense it), dry mucous membranes, restlessness, confusion, and in severe cases, seizures or coma. The danger comes from brain cells shrinking as water is pulled out of them by the high sodium concentration in the surrounding fluid. Just as with hyponatremia, the brain’s inability to adapt fast enough is what makes rapid changes so dangerous.
Sodium Levels in the Hospital and What They Predict
Abnormal sodium on a hospital admission lab panel is more than a footnote. In a large study of hospitalized patients, in-hospital mortality was about 5% among those with normal sodium levels. It rose to 9% with mild hyponatremia, 14% with severe hyponatremia, and jumped dramatically on the high side: 28% with mild hypernatremia and 52% with severe hypernatremia.14European Journal of Internal Medicine. Sodium levels on admission are associated with mortality risk in hospitalized patients Hypernatremia’s striking mortality numbers likely reflect the profile of patients who develop it: they’re often critically ill, sedated, or unable to access water independently.
Sodium fluctuations during a hospital stay carry their own risk. Among surgical intensive-care patients, having an abnormal sodium at ICU admission roughly doubled the odds of dying in the hospital, and developing a new sodium imbalance during the ICU stay carried a similar increase.15Critical Care Medicine. Fluctuations in Serum Sodium Level Are Associated With an Increased Risk of Death in Surgical ICU Patients Even in patients who arrive with normal sodium, a rise of more than 10 mmol/L in the first 48 hours of ICU care was associated with more than four times the odds of in-hospital death.16PubMed Central. Association Between an Increase in Serum Sodium and In-Hospital Mortality in Critically Ill Patients
None of this means that an abnormal sodium on a routine outpatient panel is a death sentence. The hospital data reflect acutely ill populations. But the pattern is consistent: sodium instability is a marker that something significant is going on, and it deserves prompt investigation rather than a shrug.
Exercise-Associated Hyponatremia
One setting where healthy people unexpectedly develop dangerously low sodium is endurance sports, especially marathons, ultramarathons, and long triathlons. The combination of heavy sweating, drinking large volumes of plain water or hypotonic sports drinks, and continued vasopressin release during prolonged exercise can dilute serum sodium below safe levels.17PubMed Central. Exercise-Associated Hyponatremia in Marathon Runners
A well-known study of Boston Marathon runners found that hyponatremia was strongly associated with drinking more than 3 liters during the race, gaining weight during the event (a sign of fluid overaccumulation), and finishing in over four hours. Slower runners, women, and those at the extremes of body mass were at higher risk.18PubMed. Hyponatremia among runners in the Boston Marathon The practical takeaway is that “drink as much as possible” can actually be dangerous during long exercise bouts. Current guidance for endurance athletes emphasizes drinking to thirst rather than forcing fluids on a rigid schedule, and using beverages that contain some sodium when exercise stretches beyond an hour or two.
Does Eating Salt Raise Your Serum Sodium?
This is one of the most natural follow-up questions, and the answer is more nuanced than most people expect. Eating a salty meal does temporarily nudge your serum sodium upward. One study found that plasma sodium rose by about 3 mmol/L after a salted soup compared to the same soup without added salt.19Kidney International. Dietary salt influences postprandial plasma sodium concentration and systolic blood pressure But that bump is short-lived in a person with normal kidneys and a working thirst mechanism. You feel thirsty, you drink, your kidneys adjust, and the concentration drifts back to baseline within hours.
The reason dietary salt is linked to high blood pressure but not to chronically elevated serum sodium is that the body responds to a salt load primarily by retaining water to keep the concentration steady. The result is increased blood volume and pressure, not a meaningfully higher sodium reading on a blood test. Someone with persistently high serum sodium almost certainly has a water-access or water-regulation problem, not a salt-consumption problem.
How Doctors Work Up an Abnormal Sodium
When a sodium result comes back abnormal, the next steps involve figuring out the “why,” not just the “what.” For low sodium, doctors typically check the concentration of dissolved particles in both the blood (serum osmolality) and the urine (urine osmolality). If serum osmolality is low and urine osmolality is also appropriately low, the kidneys are doing their job by trying to dump excess water, and the cause is usually excessive water intake. If urine osmolality is inappropriately high, the kidneys are holding onto water when they shouldn’t be, pointing toward vasopressin-driven conditions like SIAD.20PubMed Central. The hyponatremic patient: a systematic approach to laboratory diagnosis
Urine sodium concentration adds another layer: a very low urine sodium suggests the body is holding onto every bit of sodium it can, which usually means the person is volume-depleted (from vomiting, diarrhea, or diuretic overuse). A high urine sodium points instead toward SIAD or certain kidney diseases. Combining these lab values with a clinical assessment of the patient’s fluid status, their medications, and their symptoms narrows the diagnosis far more reliably than the serum sodium number alone.
When the Lab Number Itself Is Wrong
Before assuming a sodium result reflects what’s actually happening in the body, it’s worth knowing that false readings do occur. One of the most common causes in hospital settings is contamination of the blood sample with intravenous fluids. If blood is drawn from a line running near an IV site or while an IV pump is active, the sample gets diluted. The telltale pattern is a low sodium paired with a low hemoglobin and an oddly high glucose, since dextrose-containing IV solutions spike glucose while diluting everything else.21PubMed Central. New solutions to old problems: A practical approach to identify samples with intravenous fluid contamination in clinical laboratories – Section: Introduction Experienced clinicians know to question a result that doesn’t fit the clinical picture and to redraw from a clean site before acting on it.
Another source of artifact is extreme levels of fats or proteins in the blood (very high triglycerides or the abnormal proteins seen in multiple myeloma). These can throw off older laboratory methods that measure sodium in the total sample volume rather than in just the water portion. Modern direct ion-selective electrode analyzers have largely solved this problem, but not every lab uses them on every instrument.
Older Adults and Sodium Vulnerability
People over 65 end up on both sides of the sodium spectrum more often than younger adults, for reasons that compound each other. The aging kidney concentrates and dilutes urine less efficiently. Thirst perception declines, so dehydration develops without the normal alarm signal of feeling parched.12PubMed. Clinical aspects of changes in water and sodium homeostasis in the elderly On top of that, older adults are more likely to take thiazide diuretics, SSRIs, and other medications that independently push sodium lower. The combined result is that a minor illness, a hot day, or a medication change can tip an older person into a sodium abnormality that a younger person’s physiology would shrug off.
This vulnerability matters because the consequences land harder in this age group. The link between chronic mild hyponatremia and fall risk is particularly worrisome: the gait instability and cognitive fog it produces can be mistaken for normal aging or early dementia, leaving the treatable cause unaddressed.8PubMed Central. Is chronic hyponatremia a novel risk factor for hip fracture in the elderly? A sodium check is one of the cheapest and fastest blood tests available, and it’s something to flag for a doctor if an older family member seems more confused or unsteady than usual.
Why Sodium Abnormalities Keep Getting Studied
For decades, mild sodium derangements were treated as incidental findings, especially mild hyponatremia in the 130-to-134 range. That attitude has shifted. Research continues to link even modestly abnormal sodium levels to longer hospital stays and worse outcomes across a range of diseases. Median hospital stays are longer for patients admitted with mild hyponatremia or mild hypernatremia compared to those with normal levels.14European Journal of Internal Medicine. Sodium levels on admission are associated with mortality risk in hospitalized patients Whether correcting these mild derangements improves outcomes, or whether they’re simply markers of the underlying disease severity, is one of the open questions that keeps driving research. In the meantime, what’s clear is that the humble sodium level on a basic metabolic panel carries more clinical weight than its routine appearance might suggest.