Sodium (Na)
Serum sodium is a concentration, not a count of how much salt you ate. When it falls, the usual story is extra water diluting the blood, which is why the fix is rarely the salt shaker.
Part of the Comprehensive Metabolic Panel (CMP) — see all 17 values together, including Anion Gap, Calcium, Glucose.
A low sodium almost never means you ate too little salt. The result lands on a metabolic panel, the word "low" does its work, and reaching for the salt shaker feels obvious. It is usually the wrong instinct, because the number on your report is not a tally of how much salt is in your body. It is a concentration, and a concentration can fall two ways: less salt, or more water.
Think of the broth in a soup pot. How salty it tastes depends on two things: how much salt went in, and how much water it is dissolved in. Pour a cup of water into a well-seasoned pot and the same salt suddenly tastes bland, though not a grain left the pot. Serum sodium behaves the same way. The lab measures the saltiness of the broth, the concentration in the watery part of your blood, so the reading drops whenever water rises faster than salt. In most people who turn up with a low sodium, the salt is fine. There is simply too much water in the pot.
That single shift, from "how much salt" to "how salty," is the whole reading of this marker. Sodium is the main electrolyte in the fluid outside your cells, and MedlinePlus describes the body keeping its concentration inside a famously narrow band, with the kidneys flushing any excess out in the urine. The body defends the saltiness of the broth, not the amount of salt, so a falling sodium points at the water far more often than at the diet. Sodium balance is steered in part by aldosterone, the adrenal hormone that tells the kidneys how much salt and water to keep.
What a sodium value usually means
mmol/LThe watery part of the blood is diluted well below the normal band. StatPearls notes that symptoms depend heavily on how quickly the drop happened. A fast fall here can bring headache, nausea, confusion, and in severe cases seizures, as brain cells swell. A slow drift to the same number can be far quieter.
Below the usual floor but often without symptoms. American Family Physician notes that mild hyponatremia is common, especially in older hospital patients, and is more often found than felt. The cause still gets chased, since this is where the water-versus-salt question gets answered.
The band most labs read as normal for adults of either sex. The body holds the broth this steady on purpose; your report's printed range is the one that applies to your result.
The broth has concentrated because water was lost faster than salt, or not replaced. StatPearls ties this most often to dehydration in people who cannot drink freely, so a high sodium is read against hydration and access to water first.
The two edges of that scale tell almost opposite stories, and both are usually about water: a low reading is the pot with too much water poured in, a high reading the pot left to simmer down. Knowing that keeps a low sodium from sending someone to the salt instead of to the real question of where the extra water came from.
Why a low sodium is usually a water problem
Hyponatremia, a sodium below 135 mmol/L, is the most common electrolyte abnormality clinicians see. StatPearls frames it plainly: it develops when total body water rises out of step with the body's solute, so the same sodium spreads thinner across more fluid. The salt did not leave; the broth got watered down.
American Family Physician sorts the causes by the body's fluid status, its way of asking how much water is in the pot. The common threads:
- The body holding on to water. In SIADH, a hormone signal tells the kidneys to retain water they should be passing, so the broth dilutes. Heart, liver, and kidney disease can do the same by trapping fluid.
- Too much water coming in. Drinking far beyond what the kidneys can excrete, including the overhydration some endurance athletes reach during long events, pours water into the pot faster than it drains.
- Water-and-salt losses topped back up with plain water. Prolonged vomiting or diarrhea loses both, and replacing only the water leaves the broth thin.
There is also a version that is not a real dilution at all. AAFP describes pseudohyponatremia and the related effect of a high blood glucose, where very high lipids, protein, or sugar distort how the sample reads or pull water out of cells, so the sodium looks low without the body's water balance being off. It is the lab's view of the broth distorted, not the broth itself, and it is sorted out before anyone treats a number.
When a low sodium is felt, and when it is silent
The same value can be an emergency or a footnote depending on one thing: speed. Sodium governs how water moves into and out of brain cells, so a level that falls fast leaves the brain swollen before it can adapt, while a slow drift gives it time to adjust. StatPearls lists the severe end as seizures, obtundation, and delirium. This is why a sodium of 130 reached overnight is treated differently from a steady 130 someone has carried for months.
When sodium runs high
A high sodium is the simmered-down pot: water lost or not replaced, salt left behind, the broth concentrated. StatPearls names the mechanism as water loss outpacing solute loss, usually with too little drinking to keep up. The body's defense is thirst, which is why hypernatremia shows up most in people who cannot answer that pull alone.
StatPearls points to infants and frail older adults as the groups most affected, since both can lose the ability to feel thirst or reach water without help. Fluid lost through a stomach bug, heavy sweating, or burns concentrates the broth further when it goes unreplaced. The reading climbs not because someone ate more salt, but because the water that should dilute it never came.
If your sodium came back abnormal
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1
Talk to your doctor before changing salt or water
Because a low sodium is usually a water problem, eating more salt or drinking more water can be exactly the wrong move. The cause decides the fix, and that is a conversation for whoever ordered the test.
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2
Ask how fast the level moved
American Family Physician warns that raising a low sodium too quickly can cause osmotic demyelination, a serious brain injury, so correction is done slowly and on a schedule. A value that changed fast is read more urgently than the same value reached over weeks.
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3
Bring your full medication list
Diuretics, some antidepressants, and other drugs can shift sodium by changing how the body handles water. Tell the prescriber what you take so the result is read against them.
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4
Look at the kidney numbers and glucose beside it
A very high blood sugar can pull the sodium down on paper, and the kidneys set how much water and salt leave the body. Reading sodium next to those neighbors settles the water-versus-salt question.
Sodium makes sense next to its neighbors
Sodium rarely answers a question alone. It sits on the metabolic panel beside potassium, chloride, and carbon dioxide, the cluster describing the blood's electrolyte and acid-base balance, and the comparison of sodium and potassium covers why those two are read as a pair. Because the kidneys govern how much water and salt the body keeps, sodium is also weighed against creatinine and the rest of the kidney panel, and a high glucose is checked because it can dilute the reading without the water balance being off. Because the kidneys release renin to defend sodium balance, a sodium result is sometimes read beside it during a blood pressure workup. Sodium is the large positive charge the lab counts first when it calculates the anion gap, the leftover residual that flags unmeasured acids. The walk-through of the metabolic panel reads the whole row as one picture.
The honest takeaway is gentler than the word "low" suggests. A low sodium is a message about water, not a verdict on your salt, and the value alone says less than how fast it moved and what its neighbors show. On the printout sodium often appears only as "Na" rather than its full name, one of the shorthand codes a lab report uses that can leave a familiar test looking unfamiliar.
See your Sodium on one timeline.
BloodSight calibrates the reference range to your sex, age, and lab — and shows every value across every visit.
In your personal range
Normal ranges
| Group | Range | Unit |
|---|---|---|
| Adult Male | 135–145 | mmol/L |
| Adult Female | 135–145 | mmol/L |
Reference ranges may vary by laboratory and individual factors.
Sodium — Common Questions
Does a low sodium mean I need to eat more salt?
What is a normal sodium level?
Why would my sodium be low if I feel fine?
What does a high sodium level mean?
Why does the rate of change matter more than the number?
Do I need to fast before a sodium test?
Disclaimer
This content is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment recommendation. Reference ranges may vary by laboratory. Always discuss your results with a qualified healthcare professional.
Related Tests
Potassium sits in a famously narrow band, and a single alarming result is more often the blood draw than the body. The challenge is telling a breached test tube apart from a real shift in your kidneys.
Chloride is the body's main negative charge, and on its own it says little. Its real job is to keep the books balanced, which is how a calculation called the anion gap catches an acid nobody measured directly.
Creatinine is the muscle waste your kidneys clear. The catch is that the same number reads high in a bodybuilder and normal in someone whose kidneys are already struggling, which is why eGFR exists.
Glucose is a single photograph of your blood sugar, captured the instant the needle goes in. Whether you had eaten, the hour of day, even the stress of the draw can change what the picture shows.
Albumin is the protein that keeps water inside your blood vessels. When it falls, the number rarely means a protein-poor diet — it usually means something is being lost, made too slowly, or quietly inflamed.
The salt-and-water hormone that can quietly run high behind blood pressure no medication seems to fix.
Renin is the question; aldosterone is the answer. Read together, the pair screens for one of the most common fixable causes of high blood pressure.
No technician ever measured your anion gap. It is pure subtraction whose only job is to reveal the acids the panel was never asked to test for.