Hyponatremia is defined as a serum sodium concentration below the normal range, typically less than 135 mEq/L. Sodium is a vital electrolyte that plays a critical role in many of the body’s essential functions. It helps to maintain normal blood pressure, supports the function of nerves and muscles, and regulates the body’s fluid balance.

The body has a very delicate and tightly controlled system for maintaining this fluid and sodium balance. The key hormone involved is Antidiuretic Hormone (ADH), which is released by the pituitary gland in the brain. ADH acts as a signal to the kidneys, telling them to retain water and produce less urine.

A helpful analogy is to think of your bloodstream as a large pitcher of salty water that must be kept at a perfect level of “saltiness” to keep your body’s cells working correctly.

• The sodium is the salt in the pitcher, and your body fluid is the water.
• The hormone ADH is like a switch that controls a faucet, adding fresh water to the pitcher.
• In many of the most common forms of hyponatremia, this ADH switch gets stuck in the “on” position. The faucet keeps pouring fresh water into the pitcher, even when it is not needed.
• Even though the amount of salt in the pitcher has not changed, the constant addition of water dilutes the mixture, making it less salty. This diluted state is hyponatremia. The problem is not a lack of salt, but a relative excess of water.

This dilution has serious consequences, particularly for the brain. When the blood becomes too dilute (less salty) compared to the fluid inside the brain cells, water moves via osmosis from the blood into the brain cells to try to equalize the concentration. This causes the brain cells to swell. Because the brain is enclosed in the rigid, bony skull, there is no room for this swelling, a condition known as cerebral edema. This brain swelling is what causes the severe neurological symptoms of acute hyponatremia.

In my experience, patients often don’t realize how serious low sodium can be until symptoms like confusion or fatigue begin affecting daily life especially in older adults.

A low sodium level is the result of a disruption in the body’s normal water and sodium balance. To determine the cause, doctors categorize hyponatremia into three main types based on the patient’s total body fluid status.

1. Hypovolemic Hyponatremia (Low Fluid Volume)

In this type, the person has lost both salt and water, but they have replaced their losses with primarily plain water. This dilutes the remaining sodium in their body. This is a state of dehydration combined with low sodium.

2. Euvolemic Hyponatremia (Normal Fluid Volume)

In this type, the total amount of sodium in the body is relatively normal, but there is an excess of total body water. This is a purely dilutional state. The most common cause is the Syndrome of Inappropriate Antidiuretic Hormone (SIADH), where the body produces too much ADH, causing the kidneys to retain too much water.

3. Hypervolemic Hyponatremia (High Fluid Volume)

In this type, the body has an excess of both salt and water, but the increase in water is much greater than the increase in salt. The person is visibly swollen with fluid (edematous). This occurs in conditions where the body’s normal fluid regulation systems are failing.

Patients frequently ask how their sodium dropped. I explain that it’s often not about eating less salt, but more about water balance and how the body handles fluid.

A person develops hyponatremia due to a specific medication, illness, or activity that disrupts the normal water and sodium balance.

The most common causes and risk factors for each type include:

• For Hypovolemic Hyponatremia:
  • Use of Diuretic Medications: Thiazide diuretics (“water pills”) used for high blood pressure are a very common cause, especially in older adults.
  • Gastrointestinal Losses: Severe or prolonged vomiting or diarrhea can cause a significant loss of both salt and water.
  • Excessive Sweating: Endurance athletes, like marathon runners, who lose large amounts of salt and water through sweat and rehydrate with only plain water are at high risk.
• For Euvolemic Hyponatremia:
  • SIADH: This syndrome can be caused by a wide range of conditions, including certain lung diseases (like pneumonia or small cell lung cancer), brain disorders, and, very commonly, by certain medications, especially SSRIs (a class of antidepressants).
  • Primary Polydipsia: A condition where a person drinks excessive amounts of water, often due to a psychiatric disorder.
• For Hypervolemic Hyponatremia:
  • Congestive Heart Failure: The weakened heart cannot pump effectively, leading to fluid backup and retention.
  • Liver Cirrhosis: Advanced liver disease leads to complex fluid shifts and ascites.
  • Advanced Kidney Failure: The kidneys lose their ability to excrete excess water.

Clinically, I’ve seen patients with chronic conditions like kidney disease, liver cirrhosis, or SIADH (Syndrome of Inappropriate Antidiuretic Hormone) present with this imbalance.

The symptoms of hyponatremia depend on both the severity (how low the sodium level is) and the acuity (how quickly it developed). Chronic, mild hyponatremia that develops slowly over weeks may cause few or no symptoms. In contrast, a rapid drop in sodium over just 48 hours can be life-threatening.

Symptoms are mostly neurological, caused by the effects of brain cell swelling.

Mild to Moderate Symptoms:

• Nausea and vomiting
• Headache
• A general feeling of fatigue and lethargy
• Muscle cramps or weakness

In older adults, even chronic mild hyponatremia can cause subtle symptoms like difficulty concentrating, gait instability, and an increased risk of falls.

Severe, Acute Symptoms

A rapid and severe drop in sodium is a medical emergency. The symptoms are signs of significant cerebral edema and require immediate hospital care. These include:

• Altered Mental Status: This can range from confusion and disorientation to extreme drowsiness and unresponsiveness.
• Seizures.
• Coma.
• Respiratory Arrest: As the brainstem is compressed, the respiratory drive can fail.

Clinically, I watch for subtle signs like unsteady walking, fatigue, or headaches, which can be early indicators in elderly patients before neurological symptoms appear.

The diagnosis of hyponatremia is made with a simple blood test, but figuring out the underlying cause requires a careful and systematic investigation by a doctor, usually in a hospital setting.

• Basic Metabolic Panel (BMP): The diagnosis is confirmed with a standard blood test that measures the serum sodium concentration.
• The Diagnostic Workup: The entire focus then shifts to finding the “why.”
  • Clinical Assessment: A doctor will first assess the patient’s volume status, are they dehydrated (hypovolemic), swollen (hypervolemic), or do they appear to have a normal fluid balance (euvolemic)? This is the most important first step in narrowing the cause.
  • Medication Review: A thorough review of all the patient’s prescription and over-the-counter medications is critical, as drugs are a very common cause.
  • Further Laboratory Tests: A doctor will order additional tests on the blood and urine to pinpoint the cause.
    • Serum Osmolality: This blood test measures overall blood concentration.
    • Urine Osmolality and Urine Sodium: These urine tests are crucial for differentiating between the various causes. For example, in SIADH, the urine will be inappropriately concentrated, while in primary polydipsia, it will be very dilute.

Clinically, I confirm hyponatremia through a basic metabolic panel and then order further tests like serum osmolality and urine sodium to determine the underlying cause.

The treatment for hyponatremia depends entirely on the underlying cause, the severity of the symptoms, and how quickly the condition developed. All but the mildest cases require management in a hospital.

The Critical Rule: Avoid Rapid Overcorrection

The most important principle in treating chronic hyponatremia is that the sodium level must be corrected slowly and cautiously. If the blood sodium level is raised too quickly, it can cause a devastating and often irreversible neurological condition called osmotic demyelination syndrome (ODS). In this condition, the rapid shift in fluid out of the brain cells strips them of their protective myelin sheath. Therefore, a doctor will aim to raise the sodium level by only a small, safe amount over each 24-hour period, with frequent blood test monitoring.

Treatment by Type: The treatment strategy is tailored to the patient’s volume status.

• For Hypovolemic Hyponatremia (fluid down): The treatment is to give intravenous (IV) normal saline to replenish both salt and water.
• For Euvolemic Hyponatremia (primarily SIADH): The cornerstone of treatment is fluid restriction. By limiting free water intake, the body is able to gradually excrete the excess water and the sodium concentration will rise.
• For Hypervolemic Hyponatremia (fluid up): The treatment involves a combination of sodium restriction and diuretic medications to help the body excrete the excess fluid.

Treatment of Severe, Symptomatic Hyponatremia

In a patient who is having seizures or is in a coma from severe hyponatremia, this is a medical emergency. In an ICU setting, a doctor will administer a small, carefully calculated infusion of hypertonic saline (3% saline). This very salty solution works quickly to pull water out of the swollen brain cells to relieve the dangerous cerebral edema.

I’ve seen the best results when the treatment matches the cause whether it’s fluid restriction, salt tablets, or adjusting medications. Rapid correction can be dangerous and must be done carefully.

Hyponatremia, or a low sodium level, is the most common electrolyte disorder encountered in medicine. It is a complex condition that is most often a problem of water imbalance rather than salt deficiency. While mild cases can be subtle, a severe or rapid drop in sodium is a life-threatening emergency that can cause the brain to swell. The key to successful management lies in a careful diagnosis to determine the underlying cause and a cautious, controlled approach to correction under medical supervision. Clinically, I’ve found that treating the underlying condition whether it’s heart failure, medications, or hormone imbalances brings the best long-term control of sodium levels.

The Mayo Clinic. (2022). Hyponatremia. Retrieved from https://www.mayoclinic.org/diseases-conditions/hyponatremia/symptoms-causes/syc-20373711

The Merck Manual Professional Version. (2023). Hyponatremia. Retrieved from https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/electrolyte-disorders/hyponatremia

National Kidney Foundation. (2023). Sodium and Your CKD Diet. Retrieved from https://www.kidney.org/atoz/content/sodium-and-your-ckd-diet