Sodium · low result
Low sodium (hyponatremia): what it means, causes, symptoms and what to check next
Short answer
Low sodium (hyponatremia) — below about 135 mmol/L — is the most common electrolyte abnormality, and it is almost always a water problem rather than a salt problem: the body is holding on to more water than it should relative to its sodium. The common culprits are thiazide diuretics, antidepressants (SSRIs) and other medications, heart, liver or kidney disease, drinking far more water than the kidneys can excrete (endurance events, some diets), hypothyroidism, adrenal insufficiency, and the syndrome of inappropriate antidiuretic hormone (SIADH) triggered by illness, pain, surgery or lung and brain conditions. Mild, long-standing hyponatremia (130–134) is common in older adults and often has few symptoms; below about 125, or a fast fall, can cause confusion and seizures and is an emergency. Correction has to be gradual.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
When does this result need same-day care?
- Sodium below about 125 mmol/L, or any low value with confusion, vomiting, severe headache or a seizure — emergency care
- A low sodium that has fallen quickly from a recent normal value (within 48 hours), whatever the number — same-day care
- Low sodium in someone drinking very large volumes of water or after an endurance event with collapse — emergency care
What is the normal range for Sodium?
- age 18+: 135 – 145 mmol/L
Can reflect overhydration, certain medications (especially diuretics), or hormonal conditions.
Full reference table, citations and testing options: Sodium biomarker page.
What are the common causes of low Sodium?
- Medications — thiazide diuretics (the leading drug cause), SSRIs and other antidepressants, carbamazepine and oxcarbazepine, antipsychotics, NSAIDs, desmopressin, MDMA
- Drinking more water than the kidneys can excrete — endurance exercise with heavy water intake, 'water challenges', very low-salt diets, heavy beer drinking with little food (beer potomania)
- SIADH — vasopressin released when it should not be: pneumonia and other lung disease, brain conditions, pain, nausea, surgery, and some cancers (notably small-cell lung cancer)
- Heart failure, cirrhosis and nephrotic syndrome — the body is overloaded with water and salt but the effective circulation is low, so vasopressin stays high
- Hypothyroidism and adrenal insufficiency (low cortisol) — both hold water
- Vomiting, diarrhea and diuretic use with water replacement — sodium lost, water kept
- Chronic kidney disease and the kidneys' reduced ability to dilute urine with age
- Pseudohyponatremia — very high triglycerides or protein, or very high glucose, lower the measured or the true sodium without a water problem; the lab or clinician adjusts for these
What symptoms go with low Sodium?
- Often none when mild and long-standing
- Nausea, headache, fatigue, poor concentration, muscle cramps
- Unsteadiness, falls and confusion — particularly in older adults, where mild hyponatremia is linked to falls and fractures
- Vomiting, severe confusion, seizures and coma when sodium falls quickly or below about 120
What do clinicians usually check next?
- Repeat to confirm, and check glucose and triglycerides — a very high glucose lowers sodium by dilution and is corrected for
- Review every medication and the fluid habits of the previous days — diuretics, antidepressants, water intake, alcohol
- TSH and a morning cortisol, to exclude the two hormonal causes that are easy to miss
- Paired serum osmolality, urine osmolality and urine sodium — the three tests that classify the cause (true dilutional hyponatremia, SIADH, salt loss)
- Assessment of heart, liver and kidney function and of volume status — swollen or dry
- A chest X-ray or further imaging when SIADH has no obvious trigger, since lung and brain causes are the ones not to miss
- Correction under supervision: fluid restriction, stopping the offending drug, treating the cause; raising sodium faster than about 8–10 mmol/L in 24 hours risks permanent brain injury (osmotic demyelination), so significant hyponatremia is managed by a clinician, not at home with salt
Follow-up tests available at TestWell: TSH Test (Thyroid Stimulating Hormone) ($20.99), TSH with Reflex to Free T4 Test ($29.99), TSH + Free T4 Test ($27.99), Cortisol Test (AM) ($26.49), Glucose Test ($14.99).
How do you recheck Sodium?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Basic Metabolic Panel | Test · 8 biomarkers | $17.99 | Yes | 1–3 business days |
Common questions
Can drinking too much water lower sodium?
Yes, especially during endurance exercise or with very high fluid intake and low salt. Most hyponatremia, though, is medication- or illness-related.
What is a low sodium level?
Below the lab's lower limit, usually 135 mmol/L. 130–134 is mild and common, 125–129 moderate, and below 125 severe. How fast it fell matters as much as the number: a drop over hours is far more dangerous than the same value reached over months.
What is hyponatremia?
The medical name for a low blood sodium. Despite the name it is usually not a shortage of salt but an excess of water the body cannot get rid of — because of a medication, an illness that keeps vasopressin switched on, heart, liver or kidney disease, or intake that outstrips the kidneys. The workup is about finding which.
What are the symptoms of low sodium?
Mild hyponatremia often has none, or only nausea, headache, fatigue and poor concentration. As it worsens or falls quickly: unsteadiness, confusion, vomiting, then seizures and coma. In older adults even mild low sodium raises the risk of falls and fractures, which is why it is treated rather than ignored.
Should I eat more salt to raise my sodium?
Usually no — and in SIADH, heart failure and cirrhosis it can make things worse. Most hyponatremia is corrected by restricting fluid, stopping the responsible medication or treating the cause, under a clinician who controls how fast the level rises. Salt tablets are used only in specific situations on medical advice.
Which medications cause low sodium?
Thiazide diuretics (hydrochlorothiazide, chlorthalidone) are the leading cause; then SSRIs and SNRIs, carbamazepine and oxcarbazepine, antipsychotics, NSAIDs, desmopressin, and some chemotherapy. The first weeks after starting one of these are the highest-risk period, especially in older women.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.