- 01Sodium measures concentration
Blood sodium measures how much sodium sits in each litre of blood water, so it rises when water runs short. That is why the fix for a high result is water and finding the cause, and why cutting salt barely moves it.
A blood sodium (often printed as Na) above 145 mEq/L is high 1. The test measures concentration, how much sodium sits in each litre of the water in your blood, and thirst and a hormone called vasopressin hold that concentration in a narrow band. A high reading therefore points to the water side of the ratio. The full overview of the marker lives on the sodium page; if your result was below 135, see low sodium instead.
What counts as high
Adrogué and Madias, in the standard NEJM review, set the line plainly:
Hypernatremia is defined as a rise in the serum sodium concentration to a value exceeding 145 mmol per liter.Adrogué HJ, Madias NE. N Engl J Med 2000 1
Where an example sodium of 147 fallsmEq/L
The usual range runs from 135 to 145; the table splits it into a mid-range and a high-normal upper edge. No guideline publishes a graded mild, moderate and severe scale for high sodium, so the bands use only edges a source draws: the top of the range 1, the high-normal zone from the ARIC cohort (about 15,000 US adults followed for 25 years) 3, and the critical value labs phone through 5. Sodium is reported in mEq/L in the US and mmol/L elsewhere; for sodium the two numbers are identical.
| Band | Sodium (mEq/L = mmol/L) | What it usually means | Usual next step | Your number |
|---|---|---|---|---|
| Mid-range | 135 to 142 | Water and sodium in balance 1 | Nothing to act on | See the sodium page |
| High-normal | 143 to 145 | Inside the range; when it persists in midlife, ARIC linked it with older biological age 3 | Drink normally; look again at the next routine test | 143, 144, 145 |
| Above the range | 146 to 149 | Most often a water shortfall from low intake or water loss 1 | Usually rechecked after normal drinking; doctor review if it stays up or with symptoms | 146, 147, 148, 149 |
| High | 150 to 154 | A larger water shortfall, or a cause that needs finding 1 | Call your doctor today; same-day care with any symptoms | 150 |
| Critical | 155 and over | At or past the level most US labs phone to the doctor 5 | Emergency or same-day care | No value page; act today |
When it is urgent
Is it really high?
A single result of 146 or 147 in someone who feels well is most often a snapshot of a dry morning. Check these first, in order of how often they explain one flag:
- A morning with no water: a fasting draw after a night without drinking pushes sodium up slightly. The blood test prep guide covers drinking water before a fasting draw.
- Heavy sweating: a hard session, sauna or a hot day before the draw loses water faster than sodium 1. See blood tests for athletes.
- Alcohol the night before: alcohol can make you pass more urine, which leaves the blood a little more concentrated.
- A sample left uncapped or delayed: water can evaporate from the tube and concentrate the result.
- Very high blood sugar can hide a high sodium: sugar pulls water into the blood and dilutes the sodium reading, by about 2.4 points for every 100 mg/dL of glucose above normal in one widely used study 6. If your glucose is very high, ask whether your sodium has been corrected for it.
- Lab-to-lab differences: some labs flag at 146, others at 145; read your own report's range.
Doctors commonly repeat the sodium after a few days of normal drinking, within one to two weeks, with water allowed on the morning of the draw. If the repeat is back at 138 to 142, the first result was likely a dry morning. If it is still above 145, bring it to your doctor to find the cause. If a doctor has limited how much you drink, for example for heart or kidney failure, follow that limit and call them rather than drinking more.
A useful line to take in: "Can we check BUN, creatinine, glucose and calcium, and a urine osmolality, to see whether I am losing water or not taking enough in?" Bring a list of every medicine, especially lithium or water tablets.
What causes high sodium levels in blood
Every cause of high sodium works through one of three routes: water does not come in, water goes out faster than sodium, or sodium is added 1. Adrogué and Madias note that sustained hypernatremia mostly happens when thirst is impaired or water is out of reach 1. How the kidneys handle water and salt is covered in kidney function explained.
Common
| Cause | How it raises sodium | The clue | What confirms it |
|---|---|---|---|
| Not drinking enough | Water intake falls below daily losses 1 | Older age, illness, heat, dementia or limited mobility; healthy older men drank less than younger men after going without water, despite higher sodium 4 | High BUN (blood urea nitrogen, a waste product that rises when you are dry) and a high BUN/creatinine ratio |
| Water lost through skin or gut | Sweat, fever, vomiting and diarrhoea lose more water than sodium 1 | Recent illness, exercise in heat, dark scanty urine | Concentrated urine on a urine test: urine osmolality (how concentrated the urine is) typically above 600 to 800 mOsm/kg 2 |
| Water lost through high blood sugar | Glucose in the urine drags water out with it 1 | Thirst, frequent urination, known or suspected diabetes | High fasting glucose, HbA1c |
Less common
| Cause | How it raises sodium | The clue | What confirms it |
|---|---|---|---|
| Lithium and other medicines | Long-term lithium blunts the kidney's response to vasopressin; some water tablets (loop diuretics such as furosemide) also make you lose water 1 | Large volumes of pale urine on a medicine known to do this | Dilute urine despite high sodium 2; a medicine review with your prescriber. Do not stop or change lithium or a diuretic on your own. |
| High calcium or low potassium | Both stop the kidneys concentrating urine 1 | Thirst and frequent urination | High calcium, low potassium |
| Primary aldosteronism | Excess aldosterone (a salt-holding hormone from the adrenal glands) keeps sodium high-normal, with low potassium 2 | High blood pressure plus low potassium | Aldosterone with renin |
| Sodium gain | Salt tablets, sodium bicarbonate, salty IV fluids or tube feeds add sodium faster than water 1 | Mostly in hospital; urine sodium runs high | Urine sodium |
Must not miss
| Cause | How it raises sodium | The red flag | What confirms it |
|---|---|---|---|
| Arginine vasopressin deficiency or resistance (formerly diabetes insipidus) | The brain makes too little vasopressin, or the kidneys ignore it, so large volumes of dilute urine pour out 2 | Several litres of pale urine a day and constant thirst; dangerous fast if water access is cut off | Dilute urine with high sodium 2, then a copeptin test (a blood marker that stands in for vasopressin) or a supervised water deprivation test (does the urine concentrate when you stop drinking?) 8 |
| Hyperosmolar diabetic emergency | Very high blood sugar pulls large volumes of water out through the urine 7 | Glucose around 600 mg/dL or more, extreme thirst, drowsiness; emergency care | Glucose, sodium and osmolality in hospital 7 |
| Severe dehydration in an older or dependent adult | Thirst is blunted and water is out of reach 4 | New confusion, drowsiness or a fall; same-day care | Sodium, BUN, creatinine; hospital assessment |
Symptoms
Thirst comes first, because rising sodium is the main signal for it. Dry mouth, small amounts of dark urine and tiredness follow. As sodium climbs, water leaves brain cells, and the symptoms move to the nervous system: irritability, poor focus, then confusion, muscle twitching, seizures and, at the top end, coma 1. A sodium of 146 or 147 in an adult who can drink usually causes nothing beyond thirst. Symptoms track speed as well as level: a rise over hours does more harm than the same number reached over days, because the brain has had no time to adapt 1.
The brain fog page lists sodium among the blood tests that check the medical causes of foggy thinking, next to TSH, B12 and blood sugar. Other overlapping pages: frequent urination and muscle cramps. The salt-and-blood-pressure advice you may know is about how much salt you eat; it still applies, even though eating salt barely moves this concentration 1.
Who is most at risk
High sodium clusters at the two ends of life 1. Older adults feel less thirst 4, and dementia, a care home or limited mobility can put water out of reach. Infants cannot ask for a drink, and a bout of gastroenteritis is the usual trigger 1. Others at risk are people on lithium or loop diuretics, people with uncontrolled diabetes, and hospital patients who depend on others for fluids. In hospital, death rates of about 40 to 60 percent have been reported, mostly reflecting the serious illness behind the sodium 1. Endurance athletes in the heat see it far less often than the opposite problem, low sodium from drinking too much.
Read it with your other markers
Markers that sort the causes of high sodium
| Pattern | What it suggests | Read more |
|---|---|---|
| High sodium + high BUN/creatinine ratio | Too little water in, or water lost from skin or gut | High BUN/creatinine ratio |
| High sodium + high creatinine | Dehydration strong enough to strain the kidneys; usually rechecked once rehydrated | High creatinine |
| High sodium + very high glucose | Water lost in urine through sugar; with drowsiness, an emergency 7 | High fasting glucose |
| High sodium + high calcium | Kidneys cannot concentrate urine | High calcium |
| High-normal sodium + low potassium + high blood pressure | Possible aldosterone excess | Low potassium |
| High sodium + high chloride | The two move together in water loss | High chloride |
The high BUN/creatinine ratio page puts the key step plainly: read BUN and creatinine first, because "the two individual numbers tell you which, and that decides the next step." The metabolic panel guide and electrolytes explained show where sodium sits among the rest of the panel.
CalculatorBUN to creatinine ratio calculatorWork out the ratio from the BUN and creatinine on the same panel.Work it out What usually happens next
How a doctor usually works out a high sodium
What can bring it down
Because high sodium is a water shortfall, water is what brings it down, and the cause decides whether drinking is enough. As the sodium value pages put it, "Sodium mostly tracks water, not salt in the diet." Cutting salt does little to the concentration in a healthy adult, since the kidneys clear extra sodium 1. No supplement lowers blood sodium, and salt tablets or added salt push it the wrong way. If you have heart failure or kidney disease, or have been told to limit fluids, ask your doctor how much to drink rather than drinking more on your own.
| Change | Effect on blood sodium | Evidence type | Source |
|---|---|---|---|
| Drinking water steadily through the day | Restores the water side of the ratio when intake was the cause | Mechanistic, physiology | 1 |
| Water allowed before a fasting draw | Removes the dry-morning bump from the result | Mechanistic | 1 |
| Treating high blood sugar | Stops water being lost in the urine; the sodium reading can briefly look higher as sugar comes down, which your doctor expects | Mechanistic, measured data | 6 |
| Cutting dietary salt | Little change in a healthy adult; kidneys clear the excess | Mechanistic | 1 |
| Salt tablets or added salt | Add sodium; the wrong direction for a high result | Mechanistic | 1 |
CalculatorWater intake calculatorSee how much water your day and your heat call for; if you are on a fluid limit, your doctor's number comes first.Work it out For high-normal results, the ARIC cohort followed 15,752 adults for about 25 years. Compared with 137 to 142 mmol/L, midlife sodium above 142 went with up to about 50 percent higher odds of being biologically older and up to 64 percent higher risk of chronic disease, and sodium above 144 with about 21 percent higher risk of early death 3. That is an association, and it supports drinking enough rather than any treatment.
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Citations
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med 2000;342:1493-1499
- Seay NW, Lehrich RW, Greenberg A. Diagnosis and management of disorders of body tonicity, hyponatremia and hypernatremia: Core Curriculum 2020. Am J Kidney Dis 2020;75:272-286
- Dmitrieva NI et al. Middle-age high normal serum sodium as a risk factor for accelerated biological aging, chronic diseases, and premature mortality. eBioMedicine 2023;87:104404
- Phillips PA et al. Reduced thirst after water deprivation in healthy elderly men. N Engl J Med 1984;311:753-759
- Kost GJ. Critical limits for urgent clinician notification at US medical centers. JAMA 1990;263:704-707
- Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med 1999;106:399-403
- Umpierrez GE et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care 2024;47:1257-1275
- Fenske W et al. A copeptin-based approach in the diagnosis of diabetes insipidus. N Engl J Med 2018;379:428-439
- Chauhan K et al. Rate of correction of hypernatremia and health outcomes in critically ill patients. Clin J Am Soc Nephrol 2019;14:656-663
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
