Trimethoprim-associated hyponatremia.

Area of Science:

• Nephrology
• Infectious Diseases
• Pharmacology

Background:

• Hyponatremia is a common electrolyte imbalance.
• Differentiating diuretic-induced hyponatremia from SIADH is clinically challenging.
• Pneumocystis pneumonia is an opportunistic infection often seen in immunocompromised individuals.

Observation:

• A 28-year-old man with HIV and Pneumocystis pneumonia developed severe hyponatremia (serum sodium 117 mEq/L) on day 7 of trimethoprim-sulfamethoxazole (TMP/SMX) treatment.
• Initial treatment for suspected SIADH with fluid restriction and tolvaptan was ineffective.
• The patient presented with clinical hypovolemia, confirmed by elevated renin, aldosterone, and urinary sodium levels.

Findings:

• The hyponatremia was attributed to the diuretic effect of TMP/SMX, not SIADH.
• TMP/SMX-induced hyponatremia is likely underdiagnosed and misdiagnosed as SIADH.
• Diagnosis can be differentiated by identifying clinical hypovolemia and hormonal markers.

Implications:

• TMP/SMX-induced hyponatremia should be considered in patients on high-dose TMP/SMX, especially those with Pneumocystis pneumonia.
• Sodium chloride supplementation can manage TMP/SMX-related hyponatremia if the drug cannot be discontinued.
• This case highlights a less common cause of hyponatremia and illustrates a diagnostic approach.