Hyponatremia Associated With Standard-Dose Trimethoprim-Sulfamethoxazole Use in an Immunocompetent Patient

Area of Science:

• Internal Medicine
• Pharmacology
• Nephrology

Background:

• Trimethoprim-sulfamethoxazole (TMP-SMX) is commonly used but can cause electrolyte disturbances, particularly in immunocompromised patients.
• High-dose TMP-SMX is known to cause hyponatremia, hyperkalemia, and metabolic acidosis due to dose-dependent effects on renal tubules.

Observation:

• A 72-year-old immunocompetent woman presented with weakness and ambulatory dysfunction after initiating low-dose TMP-SMX.
• Laboratory tests revealed significant hyponatremia (sodium 125 mmol/L).
• Other causes of hyponatremia, including diuretic use and endocrine dysfunction, were excluded.

Findings:

• The patient's hyponatremia was attributed to standard-dose TMP-SMX.
• The clinical presentation mimicked the syndrome of inappropriate antidiuretic hormone secretion (SIADH).
• Treatment with intravenous saline resolved the hyponatremia and associated symptoms.

Implications:

• This case demonstrates that standard-dose TMP-SMX can induce hyponatremia in immunocompetent individuals, an uncommon but significant adverse effect.
• Physicians should consider TMP-SMX as a potential cause of hyponatremia, even in patients without compromised immunity.
• Understanding the mechanism, involving blockade of aldosterone-mediated sodium reabsorption and structural similarity to potassium-sparing diuretics, is crucial for diagnosis and management.