Body temperature is held near a set point of about 37 °C (98.6 °F) by the hypothalamus, which balances heat production and heat loss. Core temperature is highest in the late afternoon and lowest in the early morning (circadian variation), and it rises with exercise, ovulation, and stress.

Heat production

• Basal metabolism and muscle activity
• Shivering — involuntary skeletal muscle contractions that rapidly increase heat
• Hormones: thyroxine (raises metabolic rate) and epinephrine and norepinephrine (sympathetic stimulation)
• Nonshivering thermogenesis from brown fat — the main heat source in newborns

Heat conservation: peripheral vasoconstriction (keeps warm blood in the core), piloerection, curling up, adding clothing.

Heat loss

Peripheral vasodilation and sweating are the main heat-loss responses; vasoconstriction conserves heat and is not a heat-loss mechanism.

Fever (pyrexia) — the set point is raised. Infection or inflammation releases pyrogens (e.g., interleukin-1, interleukin-6, tumor necrosis factor), which stimulate prostaglandin E₂ in the hypothalamus. Antipyretics work by lowering prostaglandin production.

Hyperthermia differs from fever: the set point is unchanged, but heat gain exceeds heat loss (heat illness, malignant hyperthermia, and drug reactions such as neuroleptic malignant syndrome from antipsychotics and serotonin syndrome). Antipyretics do not help hyperthermia — physical cooling does.

Age differences

• Newborns: large surface area to body mass, little insulating fat, immature sweating, rely on brown fat — lose heat quickly and are prone to cold stress.
• Older adults: reduced shivering and vasoconstriction, reduced sweating, lower metabolic rate, less subcutaneous fat, blunted thirst and temperature perception — at risk for both hypothermia and heat illness, and may have serious infection without fever.

• Temperature by an appropriate route and device, with trends (see Vital Signs); fever is generally ≥ 38.0 °C (100.4 °F)
• Skin color, temperature, moisture; chills, shivering, sweating
• Heart rate (rises about 8–10 beats/min per 1 °C or 1.8 °F of fever; a relatively slow pulse with high fever suggests certain infections or drugs), respiratory rate, BP
• Hydration: thirst, mucous membranes, urine output
• Level of consciousness — confusion suggests severe illness, heatstroke, or hypothermia
• Environmental exposure, exertion, medications (anticholinergics, antipsychotics, diuretics, beta-blockers impair heat loss)

• Core temperature (rectal, esophageal, bladder, or pulmonary artery probe) for suspected heatstroke or hypothermia — oral and tympanic readings may be inaccurate at extremes
• For fever: CBC, blood cultures (before antibiotics), urinalysis, chest X-ray, lactate when sepsis is suspected
• For heat illness and hypothermia: electrolytes, glucose, kidney function, creatine kinase (rhabdomyolysis), coagulation studies, ECG (dysrhythmias)

Temperature thresholds

Fever

• Find and treat the cause; fever itself is a defense mechanism, so antipyretics are given mainly for comfort or when fever is harmful (e.g., heart or brain injury).

Antipyretic drug safety

Heat illness

• Heat exhaustion: heavy sweating, weakness, headache, nausea, core temperature normal to below 40 °C (104 °F), normal mental status → move to a cool place, lie down, remove excess clothing, oral fluids and electrolytes.
• Heatstroke: core temperature > 40 °C with confusion, seizures, or coma; skin hot and often dry in classic heatstroke (older adults in heat waves), though sweating may persist in exertional heatstroke. This is an emergency — cool immediately (cold-water immersion is fastest for exertional heatstroke; otherwise evaporative cooling with water spray and fans, ice packs to groin and axillae), stop cooling at about 38.5–39 °C to avoid overshoot, support airway and circulation. Antipyretics are ineffective.
• Malignant hyperthermia (a genetic reaction to volatile anesthetics or succinylcholine): rising end-tidal CO₂, muscle rigidity, tachycardia, then hyperthermia → stop the trigger, give dantrolene, cool.

Hypothermia

• Remove wet clothing and prevent further heat loss first; insulate with dry blankets; warm environment.
• Mild: passive rewarming (blankets) and active external warming (forced-air warming blankets, warm drinks if alert and able to swallow).
• Moderate to severe: active core rewarming (warmed IV fluids, warmed humidified oxygen, extracorporeal rewarming).
• Handle gently and keep horizontal — rough movement can trigger ventricular fibrillation in a cold heart.
• Do not rub or massage the limbs: this sends cold peripheral blood to the core and can cause afterdrop (a further fall in core temperature) and dysrhythmias. Warm the trunk before the extremities.

1. Recognize emergencies: confusion with high temperature (heatstroke, sepsis, meningitis), or low temperature with slowed responses — call for help and begin cooling or rewarming.
2. Fever care
   • Monitor temperature every 2–4 hours or per policy, and 30–60 minutes after antipyretics.
   • Encourage fluids (unless restricted); monitor intake and output.
   • During chills, provide a light blanket for comfort; once the temperature plateaus, remove extra blankets and heavy clothing — covering a febrile client with heavy blankets to "sweat it out" raises temperature further and increases dehydration.
   • Keep skin and linens dry; oral care; rest; reduce metabolic demand.
   • Avoid alcohol rubs (skin absorption, rapid cooling, shivering). Tepid sponging is not recommended routinely; if used, stop if the client shivers.
3. Cooling blankets (hyperthermia units): wrap extremities or pad the blanket to protect skin; monitor core temperature continuously; prevent shivering (which raises heat production and oxygen use); check skin frequently for cold injury.
4. Perioperative warming: prewarm clients before surgery, use forced-air warming and warmed IV fluids, and cover the client to keep core temperature at or above 36 °C — hypothermia increases bleeding, infection, and cardiac events.
5. Newborns: dry immediately after birth, skin-to-skin contact, hat, warm environment, warm the scale and hands (see Maternity and Pediatrics).
6. Local heat and cold applications (therapeutic use)

Check skin every 5–10 minutes; place a cloth barrier; extra caution in clients with impaired sensation or circulation (diabetes, spinal cord injury, peripheral vascular disease), infants, and older adults; avoid heat over an acute injury, a bleeding site, or a malignancy.

• Take temperature correctly; fever of 38.0 °C (100.4 °F) or higher in an infant under 3 months, an immunocompromised person, or with stiff neck, confusion, rash, or breathing difficulty needs urgent care.
• Use antipyretics at the correct dose and interval; no aspirin for children and teenagers.
• Heat safety: drink water regularly, wear light loose clothing, avoid strenuous activity in the hottest hours, never leave children or older adults in parked cars, and check on older neighbors during heat waves.
• Cold safety: layered dry clothing, hat and gloves, avoid alcohol (vasodilation increases heat loss), keep older adults' homes adequately warm.

• Temperature control center = hypothalamus
• Fever raises the set point (prostaglandin E₂); hyperthermia does not — antipyretics do not work in heatstroke
• Heat loss: radiation, conduction, convection, evaporation; vasoconstriction conserves heat
• Chill phase = shivering (temperature below new set point); flush phase = sweating
• Shivering = involuntary skeletal muscle contraction producing heat; epinephrine, norepinephrine, and thyroxine increase heat production
• Newborns: brown fat, nonshivering thermogenesis; older adults: reduced sweating, may not mount fever
• Heatstroke: > 40 °C with CNS dysfunction, often hot dry skin — rapid cooling
• No alcohol rubs; don't bundle a febrile client in heavy blankets
• Perioperative hypothermia: < 36 °C
• Hypothermia: remove wet clothing first, handle gently, no limb massage (afterdrop)
• Cold for acute injury (24–48 h), heat for chronic stiffness; watch for rebound
• Avoid aspirin in children (Reye syndrome) and NSAIDs in suspected dengue