Your body runs on a careful balance of sodium and water, and when sodium drops too low, the effects can range from mild confusion to seizures. The medical name for this is hyponatremia, defined as a blood sodium level below 135 millimoles per liter (mmol/L). It happens when sodium is lost from the body, when too much water dilutes the sodium already there, or when both occur at once.
Sodium is an electrolyte, a mineral that carries an electrical charge and helps move water in and out of your cells. When sodium levels fall, water shifts into cells and causes them to swell. That swelling is most dangerous in the brain, which is packed tightly inside the skull with little room to expand.
What Can Cause Low Sodium?
The causes fall into a few broad groups: losing sodium through the gut, kidneys, or sweat; retaining too much water; hormonal and organ problems; and medications. Many cases involve more than one of these at the same time.
Gastrointestinal losses are common. Severe vomiting or diarrhea removes sodium along with fluid. So can prolonged sweating during endurance exercise, especially when a person replaces lost sweat with plain water only.
The kidneys regulate sodium closely, and when they malfunction, sodium can fall. Certain kidney diseases, adrenal gland disorders, and diuretic medications all affect how much sodium the kidneys hold onto or release.
Water retention is a major driver. When the body holds extra water, sodium becomes diluted even if total sodium has not changed. Heart failure, liver cirrhosis, and kidney failure can all trigger this. So can drinking far more water than the kidneys can excrete, a pattern sometimes seen in endurance athletes and in people with certain psychiatric conditions.
Hormonal causes matter too. Syndrome of inappropriate antidiuretic hormone (SIADH) is a condition where the body releases too much antidiuretic hormone, causing water retention and low sodium. It can be triggered by medications, lung disease, brain injury, and some cancers.
How Do Medications Cause Low Sodium?
Medications are one of the most common and most overlooked causes. Diuretics, especially thiazide types, are a well-documented trigger. They increase sodium loss through urine, and low sodium can develop within weeks of starting them.
Antidepressants, including selective serotonin reuptake inhibitors (SSRIs), have been linked to hyponatremia, particularly in older adults. Carbamazepine, an antiseizure and mood-stabilizing drug, is another known cause. So are some antipsychotics and certain chemotherapy agents.
The pattern is not always obvious. A person may start a new medication and feel fine for days or weeks before symptoms appear. This is one reason anyone starting a diuretic or antidepressant should have sodium checked if they develop headache, nausea, confusion, or unusual fatigue.
If you take any of these medications and feel unwell, do not stop the drug on your own. Call your prescriber. Stopping suddenly can cause other problems, and the sodium issue may be manageable with a dose change or a different drug.
What Are the Symptoms of Low Sodium?
Symptoms depend on how fast sodium falls, not just how low it goes. A slow drop over weeks may cause no symptoms at all until levels are quite low. A rapid drop can cause serious symptoms at milder levels.
Early signs are easy to miss:
- Headache
- Nausea or vomiting
- Fatigue and low energy
- Muscle cramps or weakness
- Restlessness or irritability
As sodium falls further, neurological symptoms appear. Confusion, difficulty concentrating, and unsteadiness are common. In severe cases, seizures, coma, and brain swelling can occur. This is a medical emergency.
Older adults are more vulnerable. Age-related changes in kidney function and hormone regulation make sodium balance harder to maintain, and symptoms like confusion may be mistaken for dementia or normal aging.
Who Is Most at Risk?
Several groups face higher risk. People over 65 are more likely to develop low sodium, partly because of age-related kidney changes and partly because they are more likely to take diuretics and other medications that affect sodium.
Endurance athletes who sweat heavily for hours and drink large amounts of plain water are at risk. So are people with heart failure, liver disease, or kidney disease, whose bodies tend to retain water.
People with certain cancers, lung conditions, or brain injuries may develop SIADH. Those taking thiazide diuretics, SSRIs, or carbamazepine should be aware of the connection.
Hospitalized patients are also a high-risk group. Low sodium is one of the most common electrolyte abnormalities seen in hospitals, often related to medications, IV fluids, or underlying illness.
How Is Low Sodium Diagnosed?
Diagnosis starts with a blood test. A basic metabolic panel measures sodium and other electrolytes. A sodium level below 135 mmol/L confirms hyponatremia. Levels between 130 and 134 mmol/L are considered mild. Below 125 mmol/L is moderate to severe.
Once low sodium is found, the next step is figuring out why. That usually involves:
- Reviewing all medications, including over-the-counter drugs
- Checking blood volume status — whether the person is dehydrated, normal, or fluid-overloaded
- Measuring urine sodium and urine osmolality to see how the kidneys are handling sodium
- Testing thyroid and adrenal function
- Checking for conditions that cause SIADH
Sometimes the cause is clear from the history. Other times it takes several tests. The goal is to identify the underlying problem, not just treat the number.
How Is Low Sodium Treated?
Treatment depends on the cause, how low sodium is, and how fast it dropped. Mild cases may only need adjustment of medications or fluid intake. More serious cases require careful medical management.
The general approach includes treating the underlying condition, reviewing and possibly changing medications, and in some cases restricting fluid intake. In severe or symptomatic cases, sodium may be replaced intravenously under close monitoring.
Correction must be slow. Raising sodium too quickly can cause a serious neurological condition called osmotic demyelination syndrome, which can lead to permanent brain damage. Clinicians typically aim to raise sodium by no more than a set amount per day, and the exact rate depends on the individual case.
This is not something to manage at home. If you have symptoms of low sodium, especially confusion, seizures, or loss of consciousness, seek emergency care.
Can Low Sodium Be Prevented?
Not all cases are preventable, but some are. If you take a diuretic or another medication linked to low sodium, ask your doctor whether your sodium should be monitored. Periodic blood tests can catch a problem before symptoms appear.
Athletes who sweat heavily for long periods should replace fluids with drinks that contain electrolytes, not just plain water. Drinking excessive plain water without replacing sodium can dilute blood sodium dangerously.
If you have heart failure, liver disease, or kidney disease, follow your care team’s guidance on fluid and sodium intake. These conditions require careful balance, and general advice may not apply to you.
Be cautious with “hydration” advice that pushes very high water intake. For most healthy adults, drinking to thirst is enough. Forcing large amounts of water beyond thirst has no proven benefit and can be harmful in rare cases.
Frequently Asked Questions
What is the most common cause of low sodium?
There is no single most common cause, but thiazide diuretics, SIADH, and conditions that cause water retention such as heart failure and liver disease are among the leading triggers. In hospitalized patients, medications and IV fluids are frequent contributors.
Can drinking too much water cause low sodium?
Yes. Drinking more water than your kidneys can excrete dilutes the sodium in your blood. This is rare in everyday life but can happen in endurance athletes and in people with certain psychiatric conditions.
What sodium level is considered dangerously low?
Sodium below 125 mmol/L is considered moderate to severe and requires prompt medical attention. Below 120 mmol/L is a medical emergency. Symptoms and how fast the level dropped matter as much as the number itself.
Can low sodium be fixed by eating more salt?
Not usually. Many cases involve too much water rather than too little sodium, so adding salt does not correct the underlying problem. Treatment depends on the cause and should be guided by a clinician.
