Low blood sodium, called hyponatremia, happens when sodium drops below 135 mmol/L. It’s the most common electrolyte disorder, and the causes range from everyday medications and drinking too much water to serious conditions involving the heart, liver, or kidneys. Mild cases (130-134 mmol/L) often produce no symptoms at all, while severe drops below 125 mmol/L can cause confusion, seizures, and coma.
How Your Body Keeps Sodium in Balance
Sodium doesn’t work alone. Your blood sodium level is really a ratio between sodium and water in your bloodstream, so anything that adds too much water, removes too much sodium, or both can tip the balance. Your kidneys do most of the regulating, adjusting how much sodium and water leave your body through urine. A hormone called antidiuretic hormone (ADH) plays a central role: when ADH levels rise, your kidneys hold onto more water, diluting the sodium already in your blood.
This means low sodium doesn’t always come from losing sodium. In many cases, it comes from retaining too much water relative to the sodium you have. That distinction matters because the underlying cause determines whether the fix involves replacing salt, restricting fluids, or treating a completely different organ.
Medications That Lower Sodium
Drugs are one of the most common and most overlooked causes of low sodium. Two classes stand out: thiazide diuretics (water pills prescribed for blood pressure) and antidepressants. Thiazides can shift sodium levels within six hours of a single dose by forcing the kidneys to excrete more sodium than usual.
Antidepressants lower sodium through a different route. They trigger inappropriate release of ADH, causing the body to hold onto water and dilute sodium. A large Danish population study found that nearly all antidepressant classes carry this risk. Citalopram had the strongest association, raising the risk of low sodium roughly 7.8 times above baseline. Venlafaxine and mirtazapine carried a moderate risk (about 3 times baseline), and duloxetine carried the lowest among the drugs studied (about 2 times baseline). Only mianserin showed no meaningful link.
Other medications that can cause low sodium include certain seizure drugs, opioids, some diabetes medications, and cancer treatments. If your sodium drops after starting a new medication, the timing alone is often a strong clue.
Heart Failure and Liver Disease
In heart failure, the heart pumps blood less effectively. Your body interprets this as low blood volume, even though total fluid may actually be high, and responds by activating hormonal systems designed to hold onto water and salt. ADH rises, the kidneys reabsorb more water than sodium, and the net result is diluted blood. Sodium goes down even as fluid builds up, which is why people with heart failure often have both swelling and low sodium at the same time.
Liver cirrhosis follows a similar pattern. A failing liver changes pressure dynamics in blood vessels, tricking the body into thinking it needs more fluid. The kidneys respond by retaining water, diluting sodium. In both conditions, the total amount of sodium in the body may actually be normal or even high, but it’s drowned out by excess water. This is sometimes called dilutional hyponatremia.
Kidney Problems
Healthy kidneys filter and fine-tune sodium levels constantly. When they’re damaged, whether from chronic kidney disease, acute kidney injury, or conditions like nephrotic syndrome, they lose the ability to excrete water efficiently. The result is the same dilution effect seen in heart and liver disease. Salt-wasting kidney disorders do the opposite: they let too much sodium escape into the urine, directly depleting your stores.
SIADH: When Your Body Holds Too Much Water
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is one of the most common causes of low sodium in people who otherwise appear healthy, with no obvious swelling or dehydration. In SIADH, the body produces too much ADH when it shouldn’t, telling the kidneys to retain water even though sodium levels are already dropping.
SIADH isn’t a disease on its own. It’s triggered by something else. The list of triggers is long:
- Brain injuries and infections: head trauma, stroke, meningitis, brain surgery, brain tumors
- Lung diseases: pneumonia, tuberculosis, COPD, small cell lung cancer
- Medications: antidepressants, seizure drugs, opioids, certain cancer treatments
- Surgery: especially procedures under general anesthesia
In older adults, SIADH sometimes appears without an identifiable cause. In younger patients, finding the underlying trigger is more important because it’s more likely to be something treatable or serious.
Hormonal Imbalances
Two hormonal conditions deserve specific mention. Adrenal insufficiency, whether from Addison’s disease or a problem with the pituitary gland, means the body doesn’t produce enough cortisol and aldosterone. Both hormones help the kidneys hold onto sodium. Without them, sodium leaks out through urine while the body simultaneously retains too much water.
Hypothyroidism (low thyroid function) can also contribute. Thyroid hormones regulate proteins in the kidney responsible for water transport and sodium balance. When thyroid levels fall, the kidneys lose their ability to clear water efficiently, leading to retention and diluted sodium. Both conditions are identified through blood tests and treated with hormone replacement.
Fluid Loss From Vomiting and Diarrhea
Prolonged vomiting, diarrhea, or heavy sweating causes the body to lose both sodium and water. The problem often gets worse when people replace those losses with plain water or other low-sodium fluids. You’re putting water back in without replacing the sodium that left, widening the gap. Burns and bowel obstructions can do the same thing by trapping sodium-rich fluid in tissues where it can’t circulate normally.
Overhydration During Exercise
Exercise-associated hyponatremia is a well-documented risk during marathons, triathlons, and other endurance events. It happens when athletes drink far more water than they’re losing through sweat, often out of fear of dehydration or by following rigid drinking schedules that don’t account for individual needs.
The excess water overwhelms the kidneys’ ability to excrete it quickly, and ADH levels often rise during prolonged exercise, compounding the problem. Taking NSAIDs like ibuprofen before or during exercise makes things worse by further impairing the kidneys’ water-clearing ability. Symptoms can appear when sodium drops below 130 mmol/L, and severe cases with sodium plunging below 110-115 mmol/L can cause seizures and coma. Drinking to thirst rather than on a fixed schedule is the simplest prevention strategy.
Low-Solute Diets and Excessive Water Intake
Your kidneys need a certain amount of dissolved substances (solutes) from food to pull water into urine. People on extremely low-protein or low-salt diets, sometimes called “tea and toast” diets, may not give their kidneys enough material to work with. The kidneys can’t concentrate the urine properly, so water backs up in the body and dilutes sodium. A related condition, beer potomania, occurs in people who consume large amounts of beer with very little solid food, for the same reason.
Psychogenic polydipsia, a condition where people compulsively drink enormous quantities of water, can also overwhelm the kidneys’ capacity to excrete fluid. This is more common in people with certain psychiatric conditions.
How Symptoms Progress
Mild low sodium often goes unnoticed. As levels drop, symptoms typically appear in a predictable sequence: fatigue and nausea come first, followed by headache and muscle cramps. Below 125 mmol/L, confusion, irritability, and drowsiness become more likely. The most dangerous cases involve rapid drops in sodium, which can cause the brain to swell. This is a medical emergency that can lead to seizures, coma, and death.
The speed of the drop matters as much as the number. A person whose sodium gradually drifts to 125 over several weeks may feel only mildly tired, while someone whose sodium crashes to 128 over a few hours may be seriously ill. The brain has time to adapt to slow changes but not to fast ones.