All about Causes of Low Sodium
Hyponatremia develops when there is too much water in the body relative to sodium, or when the body loses more sodium than it loses water. Sodium is the main electrolyte that controls fluid movement between cells and blood vessels. When levels fall below 135 mEq/L, cells—especially brain cells—begin to swell. As the Mayo Clinic explains in its detailed hyponatremia overview, understanding the exact cause is critical because treatment differs dramatically depending on whether the problem is too much water, too little sodium, or both.
Doctors classify hyponatremia into three main types based on the patient’s fluid volume status: hypovolemic (low volume), euvolemic (normal volume), and hypervolemic (high volume). This classification, highlighted by Cleveland Clinic experts, guides diagnosis and therapy far more reliably than the sodium number alone.
How Sodium Balance Is Normally Maintained
The kidneys, brain, and hormones (especially antidiuretic hormone or ADH) work together to keep sodium tightly regulated. When you drink water or lose fluid through sweat or urine, ADH tells the kidneys how much water to retain or excrete. Any disruption in this delicate system—whether from disease, medication, or behavior—can quickly lower blood sodium.
1. Hypovolemic Hyponatremia (Low Body Fluid Volume)
This occurs when the body loses both sodium and water, but loses more sodium. Common triggers include severe vomiting or diarrhea, excessive sweating replaced only with plain water, thiazide diuretics (especially in older adults), and adrenal insufficiency (Addison’s disease). In these cases the body is truly volume-depleted and the kidneys try to conserve sodium but cannot fully compensate.
2. Euvolemic Hyponatremia (Normal Body Fluid Volume)
The most common category overall. Total body water is slightly increased while sodium is normal or mildly reduced. Leading causes include the Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) — the single most frequent cause in hospitalized patients. SIADH is often triggered by pneumonia, brain tumors, stroke, certain cancers, and many medications. Other causes in this group are hypothyroidism and glucocorticoid deficiency. StatPearls (NCBI) notes that SIADH accounts for a large percentage of unexplained cases because ADH is released inappropriately, causing the kidneys to retain water even when the body doesn’t need it.
3. Hypervolemic Hyponatremia (Increased Body Fluid Volume)
The body has too much total water and sodium, but even more water. Classic conditions include congestive heart failure, liver cirrhosis with ascites, and advanced kidney disease. In these states the kidneys perceive low effective blood volume and release ADH and aldosterone, leading to further water retention and dilution of sodium.
Medications That Frequently Cause Low Sodium
Medications are the leading outpatient cause, especially in people over 65. High-risk drugs include thiazide diuretics, SSRIs and SNRIs (antidepressants), carbamazepine and oxcarbazepine (anticonvulsants), antipsychotics, and desmopressin. MedlinePlus highlights that older adults on multiple medications are particularly vulnerable because kidney function declines with age, slowing water excretion.
Lifestyle and Behavioral Causes
Excessive water intake is a growing concern. Endurance athletes who drink only water during long events can develop exercise-associated hyponatremia. People with psychiatric conditions who drink several liters of water daily (psychogenic polydipsia) are also at risk. Even healthy individuals who follow “drink 8 glasses a day” too strictly while taking certain medications can tip into hyponatremia.
To understand how these causes actually feel in daily life, see our detailed guide on symptoms of low sodium.
Who Is at Highest Risk?
Risk is highest in adults over 65, hospitalized patients, people taking multiple medications, endurance athletes, and individuals with heart, liver, or kidney disease.
The Critical Role of the Sodium Blood Test
No matter the suspected cause, diagnosis always begins with a sodium blood test. Additional tests (urine sodium, urine osmolality, serum osmolality, TSH, cortisol) help pinpoint the exact mechanism so treatment can be targeted and safe. Early testing when mild symptoms appear can prevent progression to moderate or severe stages.
Why Accurate Cause Identification Matters
Treating the wrong cause—or correcting sodium too quickly—can cause osmotic demyelination syndrome, a serious neurological injury. That is why clinicians never treat the number in isolation; they treat the underlying cause while correcting sodium at a controlled rate (usually no more than 8–10 mEq/L per 24 hours in chronic cases).
Prevention Strategies Based on Cause
Prevention depends on the risk factor: review medications regularly with your doctor, use electrolyte drinks during prolonged exercise, limit free water when ill or on diuretics, and monitor sodium periodically if you have heart, liver, or kidney disease. The National Kidney Foundation stresses that simple awareness and periodic lab checks in high-risk individuals can prevent many hospitalizations.
Long-Term Outlook
Once the cause is identified and addressed, most people return to normal sodium levels. Chronic or recurrent cases require ongoing management of the root condition, medication adjustment, and sometimes fluid restriction. With proper care, serious complications are avoidable.
References
- Hyponatremia - Symptoms and causes - Mayo Clinic
- Hyponatremia - Cleveland Clinic
- Low blood sodium - MedlinePlus
- Hyponatremia - StatPearls - NCBI Bookshelf
- Hyponatremia (low sodium level in the blood) - National Kidney Foundation
"In the lab we see the same pattern repeatedly: an elderly patient on a thiazide diuretic and an SSRI suddenly has sodium of 122 mEq/L. The most common cause I report is medication-induced SIADH or thiazide effect. But the real art is in the follow-up tests—urine sodium >30 mEq/L with high urine osmolality almost always points to SIADH. I tell residents: always ask 'What changed?'—a new drug, a recent pneumonia, or increased water intake is almost always the answer."
Most Common Causes by Setting
Outpatient
Medications (thiazides, SSRIs)
Hospital
SIADH (pneumonia, post-op)
Elderly
Multiple meds + low solute intake
Athletes
Overhydration
Real Case from My Lab:
"A 78-year-old woman on hydrochlorothiazide and sertraline presented with confusion. Sodium was 118 mEq/L. Urine sodium was 52 mEq/L and urine osmolality 420 mOsm/kg—classic SIADH + thiazide effect. We stopped both drugs, restricted fluid, and sodium rose safely. She walked out of hospital three days later. This pattern is so common that I now automatically flag thiazide + SSRI combinations in reports."
Dr. Fernando González Carril
Consultant Pathologist, Hospital Povisa (Vigo, Spain)