A blood sodium level below 135 milliequivalents per liter (mEq/L) is considered too low. The normal range is 135 to 145 mEq/L, and anything beneath that lower boundary is classified as hyponatremia. How dangerous a low reading is depends on both the number itself and how quickly it dropped.
Severity Levels and What They Mean
Not all low sodium readings carry the same risk. Clinicians break hyponatremia into three tiers:
- Mild: 130 to 134 mEq/L. Most people at this level feel completely normal. It often shows up on routine bloodwork without any noticeable symptoms.
- Moderate: 120 to 129 mEq/L. This range typically produces nonspecific symptoms like nausea, vomiting, and headache. The lower you go within this range, the more likely you are to feel noticeably unwell.
- Severe: below 120 mEq/L. Sodium this low can cause marked confusion, seizures, and coma, especially when the drop happens quickly. Levels below 115 mEq/L are a medical emergency.
Why Speed of Onset Matters as Much as the Number
A sodium level of 125 mEq/L that developed over two weeks is a very different situation from one that dropped to 125 in a single day. When sodium falls rapidly (within 48 hours), the brain doesn’t have time to adjust. Water rushes into brain cells, causing dangerous swelling that can lead to respiratory arrest and death. This is classified as acute hyponatremia.
When sodium drifts down slowly over days or weeks, the brain gradually adapts by shifting certain molecules out of its cells to reduce swelling. That’s why someone with chronic hyponatremia at 122 mEq/L might have only mild symptoms, while someone who hits that same number overnight could be seizing. Most cases where the timeline is unclear are treated as chronic, since chronic hyponatremia is far more common.
Ironically, the treatment of chronic low sodium carries its own danger. If levels are corrected too fast (more than about 8 mEq/L in 24 hours for high-risk patients), the rapid shift can damage nerve insulation in the brain, a condition called osmotic demyelination syndrome. This is why correction in a hospital setting is done carefully and with frequent blood draws to monitor progress.
Symptoms at Different Levels
Mild hyponatremia in the 130 to 134 range rarely causes symptoms you’d notice on your own. Some research has linked even mildly low sodium to subtle problems like unsteadiness and increased fall risk in older adults, but these effects are easy to miss or attribute to something else.
Once sodium drops below about 125 mEq/L, most people experience nausea, vomiting, and headache. These symptoms aren’t unique to low sodium, which is one reason hyponatremia is often caught by blood tests rather than symptom recognition alone. Below 115 mEq/L, particularly with a rapid decline, the picture shifts to neurological symptoms: confusion, drowsiness, muscle twitching, seizures, and in the worst cases, loss of consciousness.
Common Causes of Low Sodium
Low sodium is fundamentally about the balance between water and sodium in your body. It can happen because you’re retaining too much water, losing too much sodium, or both at once. Several common scenarios tip that balance.
Certain medications are frequent culprits. Water pills (diuretics), particularly the thiazide type, are one of the most common drug-related causes. Some antidepressants can trigger the body to hold onto water by stimulating a hormone called vasopressin, which tells the kidneys to reabsorb water instead of excreting it. Anti-seizure medications and some pain drugs can do the same.
A condition called SIADH (syndrome of inappropriate antidiuretic hormone) is another major cause. In SIADH, the body continuously releases vasopressin even when it shouldn’t, causing the kidneys to retain water and dilute the blood’s sodium concentration. It can be triggered by lung infections, brain injuries, certain cancers, or medications. People with SIADH typically look normally hydrated on exam, which is part of what distinguishes it from other causes.
Heart failure, liver cirrhosis, and kidney disease all cause the body to retain fluid, diluting sodium levels. Severe vomiting or diarrhea can deplete sodium directly. And conditions like untreated hypothyroidism or adrenal insufficiency can also contribute.
Low Sodium in Older Adults
Hyponatremia is especially common in people over 65. More than 50% of patients in acute geriatric hospital wards have been found to have sodium levels below 135, and about 18% of nursing home residents have chronically low sodium. Several factors converge to make this group vulnerable: aging kidneys are less efficient at conserving sodium, older adults are more likely to take medications that affect sodium balance, and chronic illnesses that cause fluid retention become more common with age.
Even mildly low sodium in older adults matters. It’s been associated with increased falls, bone fractures, and cognitive difficulties. Because the symptoms overlap heavily with general aging complaints (fatigue, confusion, unsteadiness), low sodium in this group is frequently under-recognized.
Low Sodium From Overhydration During Exercise
Exercise-associated hyponatremia is a well-documented risk during endurance events like marathons and triathlons. It happens when athletes drink more fluid than they lose through sweat, urine, and breathing. Sports drinks don’t prevent it because they’re still far less salty than blood. The excess water dilutes blood sodium, sometimes to dangerous levels.
The problem is compounded by the body’s hormonal response to prolonged exercise. Physical exertion stimulates vasopressin release, which tells the kidneys to hold onto water. When you combine that water retention with heavy drinking during a race, sodium can drop rapidly. The Third International Exercise-Associated Hyponatremia Consensus Conference identified this combination of overconsumption of fluids plus inappropriate vasopressin release as the primary mechanism.
The practical takeaway for athletes is to drink to thirst rather than forcing fluids on a set schedule. The old advice to “stay ahead of your thirst” has been largely replaced by guidance to let your body’s signals dictate your fluid intake during prolonged exercise.
What Happens During Treatment
Treatment depends on severity and how fast sodium dropped. For mild, chronic cases, the approach is often simply identifying and addressing the cause: adjusting a medication, treating an underlying condition, or moderately restricting fluid intake. Many people with sodium in the 130 to 134 range need monitoring but no aggressive intervention.
For severe or rapidly developing hyponatremia with neurological symptoms like seizures, treatment in the emergency department involves carefully administered concentrated salt solution through an IV. The goal is to raise sodium just enough to stop the acute danger, typically aiming for a small initial bump of a few mEq/L. After that, the pace of correction is deliberately slowed to avoid osmotic demyelination. Hospital stays for severe cases often involve repeated blood draws every few hours to ensure sodium is rising at a safe rate.