The most common cause of hyponatremia, or low blood sodium, is a condition called SIADH (syndrome of inappropriate antidiuretic hormone secretion). SIADH causes the body to hold onto too much water, which dilutes sodium in the bloodstream. Hyponatremia affects roughly 30% of hospitalized patients overall, and SIADH is the single most frequent explanation.
That said, low sodium has many possible triggers, and understanding the broader picture helps make sense of why it happens and what to watch for.
How SIADH Causes Low Sodium
Your body normally regulates water balance through antidiuretic hormone (ADH), sometimes called vasopressin. When you’re dehydrated, your brain releases more ADH, which tells your kidneys to reabsorb water instead of sending it to the bladder. When you’re well hydrated, ADH drops and you produce more urine.
In SIADH, the body releases ADH even when it shouldn’t. The kidneys keep reabsorbing water despite already having plenty, and that extra water dilutes sodium in your blood. The sodium itself isn’t necessarily being lost; it’s being drowned out by excess fluid.
SIADH can be triggered by a wide range of underlying problems: certain medications, lung diseases, cancers, brain injuries, infections including HIV, and surgery. In some cases, no clear cause is found at all. It can also be multifactorial, meaning several triggers contribute at once.
Medications Are a Major Trigger
Drugs are one of the leading causes of both SIADH and hyponatremia more broadly. The worst offenders fall into a few key categories: diuretics (water pills), antidepressants, anticonvulsants, and antipsychotics.
Thiazide diuretics, commonly prescribed for high blood pressure, are the single most common drug cause of severe hyponatremia. A large population-based study found that thiazide diuretics were involved in more than one in four hospitalizations with a primary diagnosis of low sodium. The risk spikes dramatically in the first week of treatment, with one study finding a nearly 50-fold increase during that window before gradually declining.
Antidepressants are the next major culprit, particularly SSRIs and SNRIs. Taking an antidepressant roughly doubles to triples the odds of developing hyponatremia. SNRIs carry a slightly higher risk than SSRIs. These medications can trigger SIADH, causing the kidneys to retain too much water. The cases are common enough that SSRIs and SNRIs lead to the highest rate of hyponatremia-related hospitalizations among psychiatric medications.
Other Common Causes by Fluid Status
Doctors typically sort hyponatremia into three buckets based on the body’s overall fluid status, which helps narrow down the cause.
- Low fluid volume (hypovolemic): The body has lost both sodium and water, but relatively more sodium. This happens with prolonged vomiting, diarrhea, heavy sweating, or diuretic use. The kidneys try to conserve water, but sodium keeps dropping.
- Normal fluid volume (euvolemic): Total body water is slightly increased, but not enough to cause visible swelling. SIADH is the classic cause here, along with thyroid problems and adrenal insufficiency.
- High fluid volume (hypervolemic): The body is retaining both water and sodium, but water even more so. This pattern shows up in heart failure, liver cirrhosis, and kidney disease, where fluid builds up in tissues and the blood sodium gets diluted.
SIADH falls in the euvolemic category, which is one reason it can be tricky to spot. People with SIADH don’t look obviously dehydrated or swollen.
What Low Sodium Feels Like
Normal blood sodium runs between 136 and 145 mEq/L. Hyponatremia is classified as mild (130 to 135), moderate (125 to 130), or severe (below 125).
With mild to moderate drops, especially when sodium falls gradually over more than 48 hours, most people have minimal symptoms or none at all. You might feel a bit off, slightly nauseated, or have a mild headache, but nothing dramatic. The brain has time to adapt to the slow shift in fluid balance.
Severe hyponatremia, or a rapid drop at any level, is a different story. Symptoms can include confusion, extreme fatigue, muscle weakness, cramps, and in the worst cases, seizures or loss of consciousness. The speed of the drop matters as much as the absolute number. A person whose sodium falls from 140 to 128 in a few hours will feel far worse than someone whose sodium has been sitting at 128 for weeks.
How It’s Identified
Hyponatremia shows up on a basic blood test. Once low sodium is confirmed, the next step is figuring out why. Urine tests help distinguish the cause. A urine sodium level below 20 mmol/L points toward dehydration or fluid loss, while a level above 40 mmol/L is more suggestive of SIADH. Urine concentration patterns also differ: dehydrated patients produce very concentrated urine, while people with SIADH produce urine that’s more concentrated than it should be given their dilute blood.
This distinction matters because the treatments are essentially opposite. Dehydration-related hyponatremia calls for fluid replacement, while SIADH-related hyponatremia requires fluid restriction. Giving fluids to someone with SIADH would make things worse.
Why Correction Speed Matters
One of the biggest risks in treating hyponatremia isn’t the low sodium itself but correcting it too quickly. When sodium has been low for more than a day or two, brain cells adapt by shedding some of their internal solutes to prevent swelling. If sodium is then raised too fast, water rushes out of those adapted brain cells, potentially causing a serious condition called osmotic demyelination syndrome, which damages the protective coating around nerve fibers.
Current guidelines recommend raising sodium by no more than 8 mmol/L in any 24-hour period for patients at high risk. Some experts apply that stricter limit to all patients. In a large study of hospitalized patients, about 74% had their sodium corrected within this safe range. If levels rise too fast, doctors can take steps to bring sodium back down temporarily, essentially hitting a controlled pause on the correction.
This is why chronic, stable hyponatremia is often managed slowly and carefully, even when the numbers look concerning. A sodium of 122 that’s been stable for weeks is less immediately dangerous than a sodium of 130 that dropped from 140 overnight.