Furosemide Decreases Serum Sodium Levels
Furosemide (Lasix) typically decreases serum sodium levels by promoting sodium excretion in the urine, which can lead to hyponatremia, especially in susceptible patients. 1, 2, 3
Mechanism of Action
Furosemide works by inhibiting sodium and chloride reabsorption in multiple segments of the nephron:
- Primarily acts in the loop of Henle 2
- Also affects sodium reabsorption in both proximal and distal tubules 2
- Can increase sodium excretion up to 20-25% of the filtered sodium load 1
- Enhances free water clearance 1
Clinical Effects on Sodium Balance
Primary Effect: Natriuresis
- Furosemide causes a prompt increase in sodium excretion 4
- Studies show that furosemide can double the total excreted sodium mass compared to patients not using diuretics 5
- Oral furosemide solution provides even more potent natriuretic effects than tablet form 6
Risk of Hyponatremia
- Hyponatremia is a common electrolyte abnormality with furosemide therapy 1, 3
- Two mechanisms of hyponatremia can occur:
- Hypervolemic hyponatremia: More common in cirrhosis, occurs due to non-osmotic hypersecretion of vasopressin and enhanced proximal nephron sodium reabsorption 1
- Hypovolemic hyponatremia: Results from overzealous diuretic therapy causing prolonged negative sodium balance with marked loss of extracellular fluid 1
High-Risk Patient Populations
Certain patients are at increased risk for developing hyponatremia with furosemide:
- Patients with cirrhosis 1, 7
- Heart failure patients 1
- Elderly patients 1, 3
- Patients on restricted salt intake 3
Monitoring Recommendations
- Monitor serum electrolytes (particularly sodium) frequently during the first few months of therapy and periodically thereafter 3
- Watch for signs of hyponatremia: weakness, lethargy, drowsiness, restlessness, and confusion 1, 3
- Consider temporary discontinuation of furosemide if sodium falls below 125 mmol/L 1
- Diuretic therapy failure may be indicated by serum sodium less than 120 mmol/L 1
Body Composition Effects
- Furosemide primarily reduces extracellular water (ECW) 8
- A randomized controlled trial showed that furosemide administration led to:
- Reduction in body weight (-1.51 ± 0.36 kg)
- Decrease in extracellular water (-1.14 ± 0.23 L)
- Interestingly, an increase in intracellular water (0.47 ± 0.28 L), possibly due to extracellular sodium loss 8
Clinical Pearls and Pitfalls
- Pearl: Combining furosemide with metolazone can significantly enhance natriuresis and diuresis 9
- Pitfall: Excessive diuresis may cause dehydration and blood volume reduction with circulatory collapse, particularly in elderly patients 3
- Pitfall: Hyponatremia can develop insidiously - monitor patients regularly, especially those at high risk 1, 3
- Pearl: Fluid restriction is generally not necessary unless serum sodium is less than 120-125 mmol/L 1
In summary, while furosemide is an effective diuretic for managing fluid overload, it promotes sodium excretion and can lead to decreased serum sodium levels. Careful monitoring and appropriate dose adjustments are essential to minimize the risk of clinically significant hyponatremia.