What does a very low urine sodium level indicate?

From the Guidelines

A very low urine sodium level (typically less than 20 mEq/L) generally indicates that the body is actively retaining sodium, often in response to decreased effective circulating volume or dehydration. This occurs because the kidneys are working to conserve sodium and water to maintain blood pressure and organ perfusion, as seen in patients with cirrhosis and ascites, where the goal of treatment is to increase urinary excretion of sodium so that it exceeds 78 mmol/day 1. Common causes of low urine sodium include volume depletion from vomiting, diarrhea, excessive sweating, or inadequate fluid intake. It can also indicate heart failure, cirrhosis, or nephrotic syndrome, where the body perceives reduced effective circulation despite normal or increased total body volume. In these conditions, increased aldosterone and antidiuretic hormone levels signal the kidneys to reabsorb more sodium and water, as noted in the management of adult patients with ascites due to cirrhosis 1. Conversely, a high urine sodium level would suggest the kidneys are excreting excess sodium, as seen in diuretic use or salt-wasting nephropathies. When evaluating a low urine sodium, it's essential to consider the patient's clinical status, including hydration, blood pressure, edema, and other laboratory values like serum sodium and creatinine to determine the underlying cause and appropriate treatment. Key points to consider in the interpretation of urine sodium levels include:

• The use of random urinary sodium concentrations, which can be helpful when they are 0 mmol/L or 100 mmol/L but are much less helpful when they are intermediate due to lack of uniformity of sodium excretion 1
• The importance of measuring urinary sodium excretion as a parameter to follow when rapidity of weight loss is less than desired in patients with portal hypertension–related ascites 1
• The role of dietary sodium restriction (2000 mg/day [88 mmol/day]) and oral diuretics as mainstays of treatment of patients with cirrhosis and ascites 1.

From the Research

Interpreting Urine Very Low Sodium Level

A very low urine sodium level can indicate several conditions, including:

• Hypovolemia, which is a state of decreased blood volume 2
• Dehydration, which can cause a decrease in urine sodium levels
• Excessive sodium loss through other means, such as gastrointestinal loss or sweating

Causes of Low Urine Sodium

Some possible causes of low urine sodium include:

• Adrenal insufficiency, which can cause a decrease in aldosterone production, leading to decreased sodium reabsorption in the kidneys 2
• Heart failure, which can cause a decrease in renal perfusion, leading to decreased sodium excretion 3
• Nephrotic syndrome, which can cause a loss of protein-bound sodium in the urine
• Use of certain medications, such as diuretics, which can increase sodium excretion in the urine

Clinical Significance of Low Urine Sodium

A low urine sodium level can have significant clinical implications, including:

• Increased risk of hyponatremia, which can cause seizures, coma, and death if left untreated 4
• Increased risk of orthostatic hypotension, which can cause dizziness and fainting
• Decreased renal function, which can progress to chronic kidney disease or end-stage renal disease 5, 6

Diagnosis and Management

Diagnosis of the underlying cause of low urine sodium requires a comprehensive evaluation, including:

• Medical history and physical examination
• Laboratory tests, such as serum electrolyte levels and urine osmolality
• Imaging studies, such as renal ultrasound or CT scan Management of low urine sodium depends on the underlying cause and may include:
• Fluid replacement to correct hypovolemia
• Sodium supplementation to correct hyponatremia
• Medications to manage underlying conditions, such as heart failure or adrenal insufficiency 4, 2