**Abstract**

Elevated patient temperature is a common vital sign abnormality in the emergency department that can be caused either by fever or hyperthermia. Fever is a frequent presentation, most commonly caused by infections of the respiratory or urinary tracts. Other occult sources include musculoskeletal, cardiac, neurological, and intra-abdominal infections. These infections can become complicated by sepsis and septic shock, conditions with high mortality. Treatment of the febrile acutely-ill patient should begin with fluids, antimicrobials, and source control. However, if this is ineffective or if the presentation is inconsistent with infection, consideration should be given to hyperthermia, rather than fever, being the cause of the patient's elevated temperature. Several life-threatening and reversible conditions can mimic sepsis and present with elevated temperature. These mimics include toxicity from medications and illicit substances, neuroleptic malignant syndrome, malignant hyperthermia, and thyroid storm. Identification of these mimics as the source of elevated temperature can lead to earlier diagnosis and improved outcomes in these patients.

**Keywords:** fever, hyperthermia, sepsis, overdose, infection
