Low sodium levels in the body (hyponatremia) happen when there is either too much water relative to sodium or an actual loss of sodium, often due to illnesses, medications, or hormone problems. It can be mild and silent or serious and life‑threatening if it develops quickly.

What “low sodium” actually means

  • Hyponatremia usually means a blood sodium level below about 135 mEq/L.
  • Sodium helps keep the balance of water inside and outside cells and supports nerve and muscle function, including the brain.
  • When sodium is too low, water moves into cells, causing them to swell; brain cells are especially sensitive to this.

Main categories of causes

At a high level, low sodium usually comes from one (or a mix) of these:

  1. Losing too much sodium and water, then replacing mostly water.
  2. Holding on to too much water, which “dilutes” sodium.
  3. Not having enough hormones that regulate salt and water.

Doctors often think about whether the body is overall “dry” (dehydrated), “overloaded” (too much fluid), or “normal volume” but with water regulation problems.

1. Fluid loss from gut or skin

These are classic, everyday medical causes:

  • Severe or ongoing vomiting or diarrhea
    • You lose both water and electrolytes like sodium through the gut.
* If you then drink mostly plain water or very low‑salt fluids, blood sodium can drop further.
  • Heavy sweating
    • Intense exercise, working in heat, or fevers can cause significant sodium loss in sweat.
* Endurance athletes who replace sweat losses with only water are at particular risk.
  • Burns covering a large area
    • Damaged skin oozes fluid rich in salts, including sodium, which can lead to hyponatremia.

2. Drinking too much water

  • Overhydration in a short time
    • Drinking very large amounts of water faster than the kidneys can get rid of it dilutes the sodium in the blood.
* This is seen in endurance events (marathons, triathlons) and sometimes in people who drink excessively due to habit or certain psychiatric conditions.
  • Exercise‑associated hyponatremia
    • Long‑duration athletes may both sweat out sodium and over‑replace with water, creating a double hit: sodium loss plus dilution.

3. Heart, kidney, and liver problems

These conditions often cause the body to “hold” too much water:

  • Heart failure
    • The heart does not pump effectively, blood backs up, and the body responds by retaining salt and water, but the net effect often dilutes blood sodium.
  • Kidney disease or kidney failure
    • Kidneys are central for salt–water balance; when they fail, they may not excrete water properly, leading to dilutional hyponatremia.
  • Advanced liver disease (cirrhosis)
    • Cirrhosis can cause fluid buildup in the abdomen and tissues; hormonal and circulation changes trigger water retention that lowers sodium concentration.

4. Hormonal and endocrine disorders

Several hormone systems directly influence sodium levels:

  • Adrenal insufficiency (Addison’s disease)
    • Low cortisol and aldosterone reduce the body’s ability to retain sodium and regulate water and potassium, often leading to hyponatremia.
  • Hypothyroidism (low thyroid hormone)
    • In some people, reduced thyroid hormone alters kidney function and water handling, contributing to low sodium.
  • Syndrome of inappropriate antidiuretic hormone (SIADH)
    • The body makes too much antidiuretic hormone (ADH), causing the kidneys to reabsorb too much water and dilute blood sodium.
* SIADH can be triggered by lung diseases, certain cancers, brain disorders, medications, or sometimes appears without a clear cause.

5. Medications that lower sodium

A surprisingly long list of common drugs can contribute:

  • Diuretics (“water pills”)
    • Often prescribed for blood pressure or heart failure, some diuretics cause the kidneys to excrete sodium in urine, sometimes more than water, leading to low sodium.
  • Antidepressants (such as SSRIs)
    • These can increase ADH levels or otherwise affect water handling, predisposing to SIADH‑type hyponatremia.
  • Pain medications
    • Some pain medicines, including certain NSAIDs and opioids, can impair kidney water excretion or interact with ADH.
  • Other drugs and substances
    • Some anti‑seizure medications and chemotherapy agents are linked to SIADH and hyponatremia in case reports and studies.
* The recreational drug ecstasy (MDMA) is notorious for causing dangerous hyponatremia because it increases ADH and often encourages heavy water drinking in party environments.

6. “Too much water” from inside the body

Sometimes the issue is not what you drink, but how your body redistributes or retains water:

  • Postoperative states and stress
    • Surgery, severe illness, or pain can raise ADH levels, making the body retain water even when sodium is not low at first.
  • Certain lung or brain diseases
    • Pneumonia, lung tumors, head injury, stroke, or infections of the brain can trigger SIADH and excess ADH release.
  • Reset osmostat
    • In some chronic conditions (including in older adults), the body “re‑sets” the sodium level it aims for slightly lower, leading to a chronically mild hyponatremia with less dramatic symptoms.

7. Lifestyle and situational factors

  • Older age
    • Older adults are more likely to take multiple medications, have heart/kidney/liver disease, and have a reduced ability to adjust thirst and urine output, all of which increase risk.
  • Intense exercise and heat exposure
    • Long workouts, hot‑weather training, or military training can combine sweat loss, overhydration with water, and sometimes use of NSAIDs.
  • Alcohol use (especially heavy or chronic)
    • Alcohol can cause dehydration, malnutrition, and hormonal changes, and in some patterns (like “beer potomania,” where diet is very low in solutes), it can predispose to hyponatremia.

8. Why low sodium can be dangerous

  • Mild cases may cause headache, nausea, slight confusion, or low energy that are easy to overlook.
  • More severe or fast‑onset cases can lead to vomiting, severe confusion, seizures, coma, or even death due to brain swelling.
  • Rapid correction of very low sodium can itself be dangerous, so treatment is usually carefully controlled in a hospital if levels are significantly low.

9. When to worry and what to do

Because symptoms can be vague, it is important to seek medical help if:

  • You have known risk factors (heart, kidney, liver, or hormone problems, or relevant medications) and develop sudden confusion, severe headache, vomiting, or seizures.
  • You are an endurance athlete or work in extreme heat and develop confusion, severe fatigue, or nausea after heavy sweating and heavy fluid intake.

Only lab tests can confirm hyponatremia, and treatment must be tailored to the underlying cause (not just “add salt” or “drink less water”).

Information gathered from public forums or data available on the internet and portrayed here.