• Jiali Chen, Yuebo Jin, Chun Li, and Zhanguo Li.
• Department of Rheumatology & Immunology, Peking University People's Hospital, Beijing, China.
• Medicine (Baltimore). 2020 Nov 25; 99 (48): e22498.

RationaleCyclophosphamide (CY) is an alkylating agent used widely to treat cancer and autoimmune diseases. Hyponatremia is a common adverse effect of high-dose and moderate-dose of intravenous CY, but is rare in patients treated with low-dose (<15 mg/kg).Patient ConcernsA 52-year-old woman with new-onset systemic lupus erythematosus (SLE) was treated with low-dose cyclophosphamide (8 mg/kg, CY), but showed sudden headaches, disorientation and weakness. Laboratory examinations revealed severe isovolumic hyponatremia along with low-serum osmolality and high urine osmolality.DiagnosisThe acute hyponatremia was consistent with the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and was an adverse event of low-dose CY, with no evidence of endocrine, cancer, pulmonary, or cerebral abnormalities relevant to the SIADH.InterventionThe hyponatremia was resolved after the supplementation of NaCl solution.OutcomesThe hyponatremia was resolved without any complications.LessonsHyponatremia induced by low-dose CY should be recognized as an underlying life-threatening complication in clinical practice.

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