RESUMEN
WHAT IS KNOWN AND OBJECTIVE: Heparin is a commonly used anticoagulant in clinic. Persistent hyperthermia with recurrent hyponatremia caused by heparin is an extremely rare drug fever, which is difficult to judge in the early stage and is often misdiagnosed. CASE SUMMARY: A 74-year-old elderly woman was admitted to our hospital due to left hip pain with limited mobility for 9 h. She was diagnosed with a femoral neck fracture, and continuous heparin anticoagulation was initiated. On the night of surgery, the patient developed high fever with a drop in the serum sodium concentration. Based on the patient's symptoms, signs, and results of the laboratory tests, postoperative absorptive heat and infectious fever were ruled out. After heparin discontinuation, her temperature and serum sodium concentration returned to the baseline levels. WHAT IS NEW AND CONCLUSION: Heparin can cause persistent or recurrent hyponatremia and should be considered in the identification of the aetiology this condition.