Sodium: How to Read Your Blood Sodium Level

Two things to internalize.

• Low sodium is almost never a salt problem. It's a water problem. Either there's too much free water on board, or the kidneys can't get rid of it normally.
• High sodium is almost always a water deficit. Older adults with a dulled thirst response, hospitalized patients who can't reach a glass, anyone with diabetes insipidus.

That's why the reflexive "eat less salt" or "eat more salt" advice usually misses the point. Fix the water and the number tracks.

The range is tight, and most labs agree on it:

• Normal: 136–145 mEq/L (also reported as mmol/L, same number).
• Mild hyponatremia: 130–135 mEq/L. Usually asymptomatic; very common on routine labs.
• Moderate hyponatremia: 125–129 mEq/L. Headache, nausea, gait instability possible.
• Severe hyponatremia: below 125 mEq/L. Risk of seizures, cerebral edema, coma. Hospital territory.
• Mild hypernatremia: 146–150 mEq/L. Almost always reflects under-hydration.
• Severe hypernatremia: above 155 mEq/L. Confusion, seizures, brain shrinkage. Hospital territory.

Speed of change matters as much as the number. A sodium that falls from 140 to 128 in 24 hours is more dangerous than a chronic sodium that has lived at 128 for years. The brain adapts slowly. Sudden shifts don't give it time.

By the time sodium climbs above 145, the body is short on water relative to solute. Once it's in the 150s, that deficit is real.

Common causes:

• Not enough water in. Older adults whose thirst response has dulled, nursing-home residents, hospitalized patients, anyone who can't get to fluids on their own.
• Too much water out. Vomiting, diarrhea, heavy sweating, burns, fevers, osmotic diuresis from uncontrolled diabetes.
• Diabetes insipidus. The kidneys can't concentrate urine. Central (pituitary) or nephrogenic (kidney). Suspect it when urine output is high and dilute despite a high serum sodium.
• Iatrogenic. Hypertonic saline, tube feeds without free water, lithium causing nephrogenic DI.
• Actual salt loading. Rare. Usually requires deliberate ingestion or a medical infusion gone wrong.

Replacement is slow. Bring sodium down too fast and you cause cerebral edema. Rule of thumb: no more than 10 to 12 mEq/L drop per 24 hours.

Hyponatremia is the most common electrolyte abnormality in medicine and one of the most over-treated. Mild hyponatremia (130 to 135) is usually silent and dilutional.

Common causes, grouped by mechanism:

• Heart failure, cirrhosis, nephrotic syndrome. The body senses low effective circulating volume even though total fluid is overloaded, and ADH holds onto more water.
• SIADH (syndrome of inappropriate ADH). Pneumonia, CNS disease, malignancy, pain, nausea, surgery. SSRIs, carbamazepine, and opioids are the classic drug culprits.
• Thiazide diuretics. The leading medication cause. Picture an older adult on hydrochlorothiazide.
• True volume depletion. Vomiting, diarrhea, diuretics, salt-wasting nephropathy. Replacing both salt and water corrects it.
• Adrenal insufficiency. Cortisol deficiency impairs free-water excretion.
• Severe hypothyroidism. Uncommon, but classic enough to be worth checking once.
• Primary polydipsia. Drinking more water than the kidneys can pee out. Usually requires more than 15 L per day.
• Beer potomania, tea-and-toast diet. Low solute intake limits free-water excretion.

Pseudohyponatremia from severe hyperlipidemia or hyperproteinemia is uncommon now that most labs use direct ion-selective electrodes. Still worth knowing about.

One sodium value rarely tells the whole story. Patterns do.

• Stable mild hyponatremia (132 to 135) for years. Usually medication (thiazide, SSRI) or chronic SIADH. Not an emergency. Worth investigating once.
• New-onset hyponatremia. Review medications, check thyroid and cortisol, consider heart, liver, or kidney disease.
• Sodium creeping up over weeks. Usually under-hydration in older adults. Often a sentinel sign of cognitive decline or limited access to fluid.
• Sodium bouncing all over the place between visits. Diuretic dosing issues, episodic GI losses, or wildly variable water intake.

For anyone on chronic diuretics, ACE/ARB therapy, SSRIs, or with heart failure or liver disease, periodic sodium monitoring is part of routine care. Usually with each medication change, and at least annually.

• Sodium below 125 mEq/L. Hospital evaluation. Risk of seizures and cerebral edema, especially if symptomatic or rapidly developing.
• Sodium above 155 mEq/L. Hospital evaluation. Risk of brain shrinkage and intracranial hemorrhage.
• New mental status changes with any abnormal sodium. Emergency department, regardless of the absolute value.
• Symptomatic mild hyponatremia (headache, nausea, gait instability, confusion). Same-day medical evaluation.
• Persistent sodium 130–135 on routine labs. Review medications (especially thiazides, SSRIs), check TSH and morning cortisol, consider SIADH workup.
• Polyuria with high sodium. Investigate for diabetes insipidus.

• Urine sodium and urine osmolality. The single most useful pair of tests for working up hyponatremia. Distinguishes SIADH from volume depletion from primary polydipsia.
• Potassium. Sodium and potassium dysregulation often coexist (diuretics, adrenal disease).
• BUN and creatinine. Establish hydration status and kidney function.
• Glucose. Severe hyperglycemia pulls water into the bloodstream and causes "translational" hyponatremia.
• TSH and morning cortisol. Hypothyroidism and adrenal insufficiency are reversible causes of hyponatremia and easy to miss.
• Comprehensive metabolic panel. Sodium is rarely interpreted in isolation; the whole panel sets context.