Low sodium is a blood sodium level below 135 milliequivalents per liter (mEq/L). Sodium is an electrolyte — a mineral that carries an electrical charge — and it plays a central role in fluid balance, nerve signaling, and muscle function. When sodium drops too low, the medical term is hyponatremia, and it can range from a mild lab finding with no symptoms to a serious condition that affects the brain.
What Is Low Sodium?
Sodium is the main electrolyte in the fluid outside your cells. It controls how much water stays in that space and how much moves into cells. Normal blood sodium generally falls between 135 and 145 mEq/L.
When sodium falls below 135 mEq/L, water tends to shift into cells to balance the concentration. That shift is what causes most of the problems. Brain cells are especially sensitive to swelling because the skull leaves little room to expand.
Low sodium is not the same as low salt in your diet. Eating less salt does not usually cause hyponatremia in healthy people. The condition is about the balance between sodium and water in your body, and the two can drift apart for many reasons.
What Causes Low Sodium?
The most common cause is a problem with water balance rather than a true lack of sodium. In many cases, the body holds onto too much water, which dilutes the sodium that is already there.
Several conditions and situations can drive this:
- Certain medications — especially thiazide diuretics, some antidepressants, and some seizure medications
- Heart failure, liver disease, and kidney disease — each can disrupt how the body handles fluid
- Syndrome of inappropriate antidiuretic hormone (SIADH) — a condition where the body releases too much of a hormone that tells the kidneys to hold water
- Drinking excessive amounts of water — especially during endurance events or extreme exercise
- Hormone disorders — such as adrenal insufficiency or an underactive thyroid
- Severe vomiting or diarrhea — which lose sodium and fluid together
Thiazide diuretics deserve special mention. They are among the most frequently prescribed blood pressure medications, and they are a well-documented cause of low sodium, particularly in older adults. This is a recognized clinical concern that doctors monitor for when starting or adjusting these drugs.
Age matters too. Older adults are more likely to develop low sodium because of changes in kidney function, hormone levels, and the number of medications they take.
What Are the Symptoms of Low Sodium?
Symptoms depend heavily on how fast sodium drops and how low it goes. A slow decline might cause no symptoms at all, while a rapid drop can be dangerous even at mildly low levels.
When symptoms do appear, they often include:
- Headache
- Nausea or vomiting
- Confusion or trouble concentrating
- Fatigue or low energy
- Muscle cramps or weakness
- Restlessness or irritability
More serious cases can lead to seizures, loss of consciousness, or coma. This happens because brain cells swell when sodium drops, and the brain cannot expand safely inside the skull.
Many people with mildly low sodium feel completely normal. The condition is often discovered on a routine blood test rather than because someone felt unwell. That is one reason it can go unnoticed.
How Is Low Sodium Diagnosed?
Diagnosis starts with a blood test that measures sodium concentration. A result below 135 mEq/L confirms low sodium. Doctors then work to find the underlying cause, because treatment depends entirely on what is driving the imbalance.
Several tests help narrow it down:
- Blood osmolality — measures how concentrated the blood is
- Urine sodium and osmolality — shows how the kidneys are handling sodium and water
- Thyroid and adrenal hormone tests — rule out hormone-related causes
- Medication review — identifies drugs that could be responsible
Doctors also consider how quickly the sodium dropped. A level that fell over days is handled differently from one that fell over hours. This distinction matters because correcting sodium too fast can cause serious harm to the brain — a condition called osmotic demyelination. Because of this, treatment is typically slow and carefully monitored.
How Is Low Sodium Treated?
Treatment targets the underlying cause, not just the number on the lab report. If a medication is responsible, a doctor may adjust or switch it. If a condition like heart failure or SIADH is the cause, managing that condition is the focus.
For mild cases, treatment may involve adjusting fluid intake. Some people are told to drink less water. In other situations, doctors may recommend increasing sodium through diet or, in specific cases, IV fluids.
Severe cases require hospital care. Healthcare providers may use hypertonic saline — a concentrated salt solution given intravenously — but only under close monitoring. The rate of correction is carefully controlled because raising sodium too quickly can permanently damage the brain.
This is a key point: low sodium is not something to self-treat. How fast or slow it should be corrected depends on the cause, the severity, and how long the imbalance has been present. Those are clinical judgments that require blood tests and medical supervision.
Can Low Sodium Be Prevented?
Some cases cannot be prevented, especially those caused by underlying disease. But certain steps may lower the risk in everyday situations.
If you take a thiazide diuretic or another medication linked to low sodium, your doctor will likely check your sodium levels periodically, especially after starting or changing the dose. Keeping those follow-up appointments matters.
During endurance events, drinking only plain water in large amounts can dilute sodium dangerously. Sports drinks that contain electrolytes are one option, but the evidence on exactly how much to drink and when is not fully settled. The general guidance from sports medicine groups is to drink to thirst rather than forcing large volumes of fluid.
For most healthy adults, normal eating and drinking habits keep sodium in a safe range. The body regulates sodium tightly through the kidneys and hormones. It takes a significant disruption — illness, medication, or extreme fluid intake — to push it out of balance.
What Is a Normal Sodium Level Compared to Low?
The table below shows how blood sodium levels are generally categorized. These ranges are standard reference values used in clinical practice.
| Category | Sodium Level (mEq/L) |
|---|---|
| Normal | 135–145 |
| Mildly low | 130–134 |
| Moderately low | 125–129 |
| Severely low | Below 125 |
These categories help doctors decide how urgently to act. But the numbers alone do not tell the full story. Someone with a sodium of 128 that dropped over a week may feel fine, while someone with the same number that dropped over a few hours could be seriously ill.
The speed of the change often matters more than the absolute number.
Frequently Asked Questions
What is a dangerously low sodium level?
A sodium level below 125 mEq/L is generally considered severe and requires prompt medical attention. Levels below 120 mEq/L can be life-threatening and typically require hospital care.
Can drinking too much water cause low sodium?
Yes. Drinking very large amounts of water in a short time can dilute blood sodium faster than the kidneys can remove the excess water. This is most often seen during endurance events but can also happen with certain psychiatric conditions or extreme fluid intake.
Does low sodium mean I need more salt in my diet?
Not usually. Most low sodium cases are caused by too much water in the body rather than too little salt. Adding salt without addressing the underlying cause can be ineffective or even harmful, so treatment should be guided by a doctor.
Can low sodium fix itself?
Mild cases caused by temporary issues like vomiting may resolve on their own once the trigger passes. But persistent or unexplained low sodium needs medical evaluation, because it can signal an underlying condition that will not resolve without treatment.
