Pathophysiology
Fever Mechanism
How pyrogens raise the hypothalamic set point — and when fever helps or harms
Fever is a regulated rise in core body temperature driven by the hypothalamic preoptic area. Exogenous pyrogens (LPS from gram-negative bacteria, gram-positive teichoic acid, viral RNA) activate macrophages to release endogenous pyrogens — IL-1, IL-6, TNF-alpha, and interferons. These cross the organum vasculosum of the lamina terminalis, induce COX-2 expression, and PGE2 raises the hypothalamic set point. The body then mounts cold-defense responses — vasoconstriction, shivering, behavioral warming — until core temperature reaches the new set point. Normal temperature is 36.5-37.5 °C orally; fever is ≥38.0 °C; hyperpyrexia ≥41.5 °C. Antipyretics (acetaminophen, NSAIDs) inhibit COX. Fever differs fundamentally from hyperthermia (heat stroke) — set point is unchanged, body simply cannot dissipate heat.
- Normal temperature36.5-37.5 °C oral; 0.5 higher rectal
- Fever threshold≥38.0 °C oral or ≥38.3 °C rectal
- Endogenous pyrogensIL-1, IL-6, TNF-α, interferons
- Final mediatorProstaglandin E2 in hypothalamus
- Hyperpyrexia≥41.5 °C; cellular damage
- Antipyretic dosingAcetaminophen 15 mg/kg q4-6h; max 4 g/day adult
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Why fever physiology matters
- Sepsis triage. Time-sensitive recognition saves lives.
- Pediatrics. Neonatal fever requires full workup; older infants risk-stratify.
- Oncology. Neutropenic fever is a medical emergency.
- Drug toxicity. Distinguish hyperthermia syndromes from fever.
- Surgery. Postoperative fever timing suggests cause (5 W's — wind, water, walking, wound, wonder drugs).
- Travel medicine. Fever post-travel has a different differential.
- Comfort care. Treat the patient, not the number.
Common misconceptions
- Treating every fever. Mild fever may aid host defense; treat for comfort.
- Calling NMS or serotonin syndrome a fever. They are hyperthermia — antipyretics fail.
- Aspirin in children with viral illness. Reye syndrome.
- Cold sponging hyperpyrexia. Triggers shivering; use cool IV fluids and ice packs to groin/axillae.
- Equating fever absence with health. Elderly and immunosuppressed may not mount fever in serious infection.
- Empiric antibiotics for FUO. Obscures diagnosis without improving outcomes.
Frequently asked questions
How does fever differ from hyperthermia?
Fever is a regulated rise — the hypothalamic set point is elevated, and the body actively defends a higher temperature. Antipyretics work. Hyperthermia is unregulated — set point is normal but heat dissipation fails. Examples — heat stroke, malignant hyperthermia (succinylcholine, halothane; treat with dantrolene), neuroleptic malignant syndrome (antipsychotics; bromocriptine, dantrolene), serotonin syndrome (cyproheptadine), thyroid storm. Antipyretics do not work; cool aggressively with ice packs, cold IV fluids.
Is fever helpful or harmful?
Both. Helpful — many pathogens grow optimally at 37 °C; raising temperature suppresses replication. Neutrophil and lymphocyte function improve; iron sequestration starves bacteria. Animal studies show worse outcomes when fever is suppressed in infection. Harmful — increases metabolic demand 10-12% per °C, worsens heart failure, triggers seizures in children. Above 41 °C protein denaturation begins. Treatment for comfort, not lower mortality.
When should fever be evaluated urgently?
Neonates <60 days with T ≥38 — full sepsis workup including LP, empiric antibiotics. Neutropenic fever (ANC <500) — empiric piperacillin-tazobactam or cefepime within 1 hour. Post-splenectomy or sickle cell patients. Returning travelers — malaria, typhoid, dengue. Immunosuppressed transplant or HIV. Fever with rash plus hypotension — toxic shock or meningococcemia. Fever with neck stiffness — meningitis, LP urgently. Persistent fever >3 weeks unexplained — fever of unknown origin workup.
How do antipyretics work?
Acetaminophen (paracetamol) inhibits central COX, lowering hypothalamic PGE2. NSAIDs (ibuprofen, naproxen, aspirin) inhibit peripheral and central COX-1 and COX-2. Aspirin contraindicated in children with viral illness — Reye syndrome (encephalopathy and liver failure). Steroids reduce IL-1, IL-6 production but mask infection. Alternating acetaminophen and ibuprofen has marginal benefit and risks dosing errors. Cool sponging treats hyperthermia, not fever; can cause shivering that worsens fever.
What causes febrile seizures?
2-5% of children aged 6 months to 5 years, often during rapid temperature rise rather than at peak. Simple — generalized, <15 minutes, no recurrence in 24 hours; benign. Complex — focal, prolonged, or recurrent; needs further workup. Usually no antiepileptic needed. Antipyretics do not prevent recurrence in trials but reduce discomfort. Reassure parents — risk of epilepsy only slightly elevated for simple seizures.
What's fever of unknown origin?
Fever >38.3 °C lasting >3 weeks without diagnosis after appropriate workup. Major categories — infections (TB, endocarditis, abscess, osteomyelitis, HIV, CMV), malignancy (lymphoma, leukemia, renal cell, hepatocellular), autoimmune (giant cell arteritis, adult Still, lupus, vasculitis), miscellaneous (drug fever, factitious, hyperthyroidism, PE, Crohn). Workup — cultures, HIV, TB, imaging, autoimmune panel, sometimes bone marrow biopsy or PET. Empiric antibiotics not recommended.
How is sepsis defined?
Sepsis = life-threatening organ dysfunction from dysregulated host response to infection. Septic shock = sepsis with vasopressor requirement plus lactate >2 despite resuscitation. qSOFA — RR ≥22, altered mental status, SBP ≤100 (any 2 = high mortality risk). Hour-1 bundle — lactate, blood cultures, broad-spectrum antibiotics within 1 hour, 30 mL/kg crystalloid for hypotension or lactate >4, vasopressors for MAP <65. Norepinephrine first-line.