Low sodium on a blood test means your sodium level has fallen below the normal range of 136 to 145 mEq/L. The medical term for this is hyponatremia, and it’s one of the most common electrolyte abnormalities found in routine bloodwork. It can range from a minor, easily correctable imbalance to a sign of a serious underlying condition, depending on how low the number is and how quickly it dropped.
What the Numbers Mean
Sodium is measured in milliequivalents per liter (mEq/L). A result between 136 and 145 is considered normal. Anything below 136 is classified as low, but severity matters a great deal:
- Mild: 130 to 134 mEq/L
- Moderate: 125 to 129 mEq/L
- Severe: Below 125 mEq/L
A reading of 133, for example, is only slightly below normal and may not cause any noticeable symptoms. A reading of 120 is a medical emergency. The distinction between these categories shapes everything about how your doctor responds, from simply rechecking the level in a few weeks to hospitalizing you for treatment.
Why Sodium Matters in Your Body
Sodium does more than you might expect. It helps regulate blood pressure, controls the balance of water moving in and out of your cells, and supports nerve and muscle function. Your kidneys are the main regulators, constantly adjusting how much sodium leaves your body through urine. A hormone called antidiuretic hormone (ADH, also known as vasopressin) works alongside the kidneys by controlling how much water your body holds onto.
When sodium drops too low relative to the amount of water in your blood, the fluid balance around your cells shifts. Water moves into cells to try to equalize the concentration, causing them to swell. This is particularly dangerous in the brain, because the skull doesn’t allow room for expansion. In acute cases, the resulting brain swelling can cause seizures, loss of consciousness, or worse.
Common Causes of Low Sodium
Low sodium doesn’t always point to one specific problem. It’s more like a signal that something is off with your body’s fluid or hormone balance. The most common culprits fall into a few categories.
Medications are among the most frequent causes. Diuretics (water pills), certain antidepressants, and some pain medications can interfere with the kidney and hormone processes that keep sodium levels stable. If you started a new medication in the weeks before your blood test, that’s worth flagging.
Heart, kidney, or liver disease can cause your body to retain excess fluid. When fluid builds up, it dilutes the sodium already in your blood, pushing the concentration down even though you haven’t actually lost sodium.
Hormonal imbalances play a significant role. Low thyroid hormone can lower blood sodium. A condition called SIADH, where the body produces too much antidiuretic hormone, causes the kidneys to hold onto water instead of excreting it. That extra water retention dilutes your blood and drops sodium levels. In some cases, SIADH also causes the kidneys to release more sodium into urine, compounding the problem.
Dehydration from vomiting or diarrhea seems counterintuitive, since you’re losing fluid, but it works because you lose sodium in those fluids and your body compensates by increasing water retention through ADH.
Drinking too much water can overwhelm your kidneys’ ability to excrete it. This is especially common during endurance events like marathons and triathlons, where heavy sweating depletes sodium while athletes replace it with plain water. But it can also happen outside of exercise if someone habitually overhydrates.
What Low Sodium Feels Like
Mild hyponatremia often produces no symptoms at all, which is why many people first learn about it from a routine blood test. When symptoms do appear, they tend to be vague and easy to attribute to other things: nausea, headache, fatigue, muscle cramps, or a general feeling of being “off.”
As levels drop further, the symptoms become more neurological. Confusion, difficulty concentrating, and irritability can develop as the brain’s cells begin to swell. Severe cases, particularly when sodium drops rapidly, can cause seizures, loss of consciousness, and brain herniation, where the swollen brain pushes against the skull. The speed of the drop matters as much as the number itself. A gradual decline over weeks gives the brain time to adapt by releasing water and certain molecules from its cells to reduce swelling. A sudden drop over hours doesn’t allow for that adaptation, making it far more dangerous even at the same sodium level.
What Happens After a Low Result
A single low sodium reading usually triggers additional testing. Because the symptoms of hyponatremia overlap with many other conditions, a physical exam alone can’t confirm or explain it. Your doctor will likely order urine tests alongside repeat blood tests to figure out what’s driving the imbalance. Urine sodium and concentration levels help distinguish between causes: for example, whether your kidneys are losing too much sodium or your body is holding onto too much water.
Your medical history, current medications, and any recent changes in fluid intake are all part of the diagnostic picture. If you’ve recently had a stomach illness, started a new prescription, or changed your drinking habits, those details matter.
How Low Sodium Is Treated
Treatment depends entirely on the cause and severity. For mild cases linked to a medication, the fix may be as simple as adjusting the dose or switching to a different drug. If excess water intake is the issue, you may be asked to limit how much fluid you drink each day.
When an underlying condition like heart failure, liver disease, or a thyroid problem is responsible, treating that condition is the primary goal. Sodium levels often improve once the root cause is managed.
Moderate to severe cases, especially those that developed quickly, require closer monitoring and sometimes hospital-based treatment. The correction has to be carefully controlled. Raising sodium too fast can cause a separate and serious neurological complication, so doctors typically aim for a slow, steady increase. You can expect frequent blood draws during this period to track your levels in real time.
For people with SIADH or chronic low sodium that doesn’t respond to simpler measures, longer-term strategies like ongoing fluid restriction or medications that help the kidneys excrete excess water may be necessary.
Who Is Most at Risk
Older adults face a higher risk of hyponatremia for several reasons. Kidney function naturally declines with age, making it harder to regulate sodium. Older adults are also more likely to take medications that affect sodium levels, particularly diuretics for blood pressure. On top of that, the thirst mechanism becomes less reliable with age, which can lead to fluid imbalances in either direction.
Endurance athletes, people with chronic illnesses affecting the heart, liver, or kidneys, and anyone taking multiple medications should be aware of the risk. If you’ve had one low sodium result, your doctor may want to recheck your levels periodically to make sure the issue has resolved or isn’t worsening.