Neoplastic Fever: This is often seen in patients with advanced cancer, particularly lymphoma or leukemia. The fever can be due to the tumor itself or due to the body's immune response to the tumor.

Autoimmune Disorders: Conditions such as rheumatoid arthritis, lupus, or vasculitis can cause high-grade fevers due to the body's immune system attacking its own tissues.
Medication-Induced Fever: Certain medications, including antibiotics, antihistamines, and anesthetics, can cause fever as a side effect.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, characterized by high fever, tachycardia, and other systemic symptoms.
Adrenal Insufficiency: This condition, where the adrenal glands do not produce enough cortisol, can lead to fever among other symptoms.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic high blood pressure, fever, and other symptoms. Missing this diagnosis can be fatal due to the risk of hypertensive crisis.
Hemorrhagic Stroke: While not typically thought of as a cause of fever, a hemorrhagic stroke can present with fever, especially if there is associated infection or inflammation.
Subarachnoid Hemorrhage: Similar to hemorrhagic stroke, this condition can present with fever, and missing the diagnosis can have severe consequences.

Familial Mediterranean Fever: A genetic disorder that causes recurrent episodes of fever, often accompanied by abdominal, chest, or joint pain.
Hyperthermia due to Malignant Hyperthermia or Neuroleptic Malignant Syndrome: These are rare but life-threatening conditions that can occur in response to certain medications or anesthetics.
Eosinophilia-Myalgia Syndrome: A rare condition characterized by high fever, muscle pain, and elevated eosinophil count, often associated with certain medications or toxins.