What is the Main Cause of Low Sodium Levels? An Expert Guide to Hyponatremia

December 25, 2025 •

2 min read

Studies show that low sodium levels, known as hyponatremia, are the most common type of electrolyte imbalance, affecting a significant portion of hospitalized patients. This condition primarily results from an imbalance of fluid and sodium, with dilution being the most frequent trigger.

Quick Summary

Hyponatremia is most often caused by excessive fluid in the body that dilutes sodium, though excessive sodium loss also contributes. Underlying diseases, medications, and overhydration are common culprits.

Key Points

Dilution is Key: The most common cause of hyponatremia is excessive fluid in the body that dilutes sodium levels, not simply a lack of dietary salt.
Underlying Diseases: Chronic conditions such as heart failure, liver cirrhosis, and kidney disease are major contributors to fluid retention and subsequent low sodium.
Medication Effects: Several medications, including diuretics, antidepressants, and certain pain relievers, can disrupt the body's sodium balance.
Fluid and Electrolyte Loss: Severe vomiting, diarrhea, or intense sweating, especially when rehydrating with only water, can cause hyponatremia.
SIADH: The Syndrome of Inappropriate Antidiuretic Hormone (SIADH) causes the body to retain water inappropriately, leading to a dilutional effect on blood sodium.

The Dilutional Effect: Why Excess Water is a Primary Cause

Often, the main cause of low sodium levels (hyponatremia) is an excess of water in the body that dilutes the sodium concentration, rather than a lack of dietary salt. This dilutional effect is a common mechanism and is frequently associated with serious health issues.

Medical Conditions Leading to Dilution or Loss

Several medical conditions can disrupt the body's fluid and sodium balance, contributing to hyponatremia. These include organ failure affecting the heart, liver, or kidneys, which impairs fluid regulation and leads to diluted sodium. Syndrome of Inappropriate Antidiuretic Hormone (SIADH) causes excessive water retention due to increased ADH production. Hormonal issues like adrenal insufficiency and hypothyroidism also disrupt sodium and water balance. Additionally, severe fluid loss from prolonged vomiting or diarrhea can lead to a significant loss of both water and sodium; if fluids are replaced mainly with plain water, sodium can become diluted relative to water.

Medications and Lifestyle Factors

Certain medications and lifestyle choices can also play a role in developing hyponatremia.

Medications: Diuretics, particularly thiazide types, can increase sodium excretion. Some antidepressants (SSRIs) and pain relievers (NSAIDs) have also been linked to sodium imbalance.
Excessive Water Intake: Drinking large amounts of water, especially during endurance activities, without replacing electrolytes can lead to dilution. Recreational drugs like MDMA can also cause severe hyponatremia.

Classifying Hyponatremia by Fluid Status

Understanding the body's fluid volume helps determine the cause of hyponatremia. Doctors categorize hyponatremia based on whether the fluid volume is low, normal, or high.


Feature	Hypovolemic Hyponatremia	Euvolemic Hyponatremia	Hypervolemic Hyponatremia
Body Fluid Volume	Decreased (both sodium and water lost, but more sodium)	Normal (excess water without sodium loss)	Increased (both sodium and water gained, but more water)
Causes	Severe vomiting or diarrhea, excessive sweating, diuretic use, Addison's disease.	SIADH, excessive water intake (psychogenic polydipsia), severe pain, certain medications, hypothyroidism.	Heart failure, liver cirrhosis, kidney disease (nephrotic syndrome), excessive water intake.
Mechanism	Net loss of total body sodium and water, with sodium loss being disproportionately larger.	Body retains excess water due to inappropriate ADH release, diluting blood sodium levels.	Body retains both water and sodium, but the fluid retention is significantly greater, causing dilution.

Increased Risk Factors

Certain individuals have a higher risk of developing hyponatremia, including older adults who are more prone to chronic diseases and multiple medications, and endurance athletes who may overhydrate with plain water.

Conclusion

The main cause of low sodium levels is most often an imbalance in the body's fluid and sodium balance, frequently due to excessive water retention and dilution, rather than insufficient salt intake. However, significant sodium loss can also contribute. Underlying medical conditions, certain medications, and overhydration are key factors. Diagnosis involves blood and urine tests to identify the specific cause. If you experience symptoms like headache, confusion, or fatigue, seek medical attention. For further information, {Link: Mayo Clinic https://www.mayoclinic.org/diseases-conditions/hyponatremia/symptoms-causes/syc-20373711} provides a comprehensive overview of hyponatremia.

Frequently Asked Questions

The most frequent reason is a relative excess of water in the body, which dilutes the sodium concentration. This is often linked to chronic diseases like heart failure or kidney disease.

Yes, drinking excessive amounts of water, especially during endurance exercise, can dilute the blood's sodium content and lead to hyponatremia. This is known as water intoxication.

Common medical conditions that cause hyponatremia include congestive heart failure, liver cirrhosis, kidney disease, and Syndrome of Inappropriate Antidiuretic Hormone (SIADH).

Yes, several medications, including certain diuretics (water pills), antidepressants (SSRIs), and pain medications, can interfere with the hormonal and kidney processes that regulate sodium.

Severe vomiting and diarrhea cause the body to lose both sodium and water. If lost fluid is replaced with plain water instead of an electrolyte solution, the remaining sodium becomes diluted, leading to hyponatremia.

Key symptoms can include nausea, vomiting, headache, confusion, fatigue, and muscle weakness or cramps. In severe cases, seizures and coma can occur.

Risk factors for hyponatremia include older age, having chronic health conditions (like kidney or heart disease), using certain medications, and participating in intense physical activity with improper rehydration.