Abstract Introduction Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is an important cause of hyponatremia that can also occur with antimicrobial therapy. While trimethoprim-sulfamethoxazole (Bactrim) is widely used, it can also induce SIADH, a complication that is often underrecognized. This case highlights the need for clinicians to be aware of this rare adverse effect to ensure timely diagnosis and management. Case Description A 76-year-old male with a medical history of dementia and hypertension, presented with a 3-day history of worsening confusion. His wife reported no recent trauma, falls, smoking, or thyroid disease. Baseline mental status over the prior six months had been consistent with mild cognitive impairment, with symptoms being more pronounced recently, leading to a formal diagnosis of dementia. On presentation, he was alert and oriented to person ,and place but exhibited word repetition and confusion. Physical examination revealed euvolemia with negative orthostatic vitals. Notably, he had multiple sebaceous cysts recently incised and drained. He had been started on trimethoprim-sulfamethoxazole (TMP-SMX) following this procedure. Initial labs revealed profound hyponatremia with a serum sodium of 119 mmol/L. Urine studies showed a urine sodium of 49 mmol/L and urine osmolality of 518 mOsm/kg, and with a serum osmolality of 277 mOsm/kg, consistent with SIADH. No other causes of hypo-osmolality were identified. TMP-SMX was then discontinued, and patient was placed on fluid restriction (1500 mL/day), resulting in a modest increase in serum sodium to 121 mmol/L. After administration of 1L of fluid, however, his Na dropped back to 119. A more aggressive fluid restriction (1000 mL/day), led to gradual sodium correction over the following 48 hours. His mental status improved, and he was transferred out of the ICU and eventually discharged home in stable condition. Discussion While commonly reported causes of SIADH include SSRIs, carbamazepine, cyclophosphamide, and vincristine, trimethoprim-sulfamethoxazole is a less recognized but potential contributor. TMP-SMX is typically associated with adverse effects such as rash, cytopenias and hyperkalemia. SIADH-related hyponatremia, however, is uncommon and often overlooked. The concurrent relationship with TMP-SMX initiation and improvement after its discontinuation in our patient, supports drug-induced SIADH as the cause. Clinicians should maintain a high index of suspicion for drug-induced SIADH in patients presenting with hyponatremia, particularly when receiving medications such as Bactrim, to ensure timely diagnosis and appropriate management. This abstract is funded by: none
Jamal et al. (Fri,) studied this question.