The human body maintains a core temperature of around 36.5–37.5°C through hypothalamic regulation, balancing heat production and loss. Disruption of this balance leads to hyperthermia (excess heat) or hypothermia (excess heat loss). Both conditions can be life-threatening and require prompt recognition and treatment.

🧬 Pathophysiology

Hyperthermia: Occurs when heat generation (e.g., exertion, drugs, endocrine excess) or environmental exposure exceeds the body’s capacity for heat dissipation. Unlike fever, the hypothalamic set point is not raised.
Hyperpyrexia: An extreme fever (>41°C) due to a raised hypothalamic set point, usually from infection, CNS disease, or drug-induced syndromes (e.g., NMS, serotonin syndrome).
Hypothermia: Results from prolonged heat loss via radiation, convection, evaporation, or conduction. Risk factors include exposure, impaired thermoregulation (age, alcohol, hypothyroidism), and metabolic derangements.

🩺 Clinical Features

Hyperthermia: Hot, flushed skin, tachycardia, confusion, seizures in severe cases. Red flag: Heatstroke → altered mental status + core temp >40°C.
Hyperpyrexia: Very high fever with rigors, delirium, or neurological dysfunction. May herald sepsis, intracranial bleed, or drug toxicity.
Hypothermia: Shivering (mild), slurred speech, confusion, bradycardia, hypotension. Severe hypothermia (<30°C) → arrhythmias (J-waves on ECG), coma, cardiac arrest.

📊 Comparison Table

Condition	Definition	Causes	Treatment
🌡️ Hyperthermia	Core temperature >38°C due to failure of heat regulation (not a raised set point).	Heatstroke (environmental exposure) Exertional heat illness Medications: anticholinergics, diuretics, stimulants Endocrine: hyperthyroidism, phaeochromocytoma Dehydration	Immediate cooling (move to shade, remove clothing) Oral/IV rehydration Active cooling: cold packs (axillae, groin, neck), cool water sponging Severe cases: immersion cooling or cooling blankets Treat underlying cause Monitor electrolytes and cardiac rhythm
🔥 Hyperpyrexia	Extreme elevation >41°C, usually due to infections or CNS causes.	Severe infection/sepsis CNS haemorrhage/trauma Neuroleptic malignant syndrome Serotonin syndrome Malignant hyperthermia (anaesthesia)	Aggressive cooling (ice packs, cooling blankets, cold IV fluids) Antipyretics (e.g., paracetamol if infection-related) IV fluids for support Treat underlying cause (antibiotics, dantrolene for malignant hyperthermia, bromocriptine for NMS) Consider sedation/intubation in refractory cases
❄️ Hypothermia	Core body temperature <35°C due to heat loss.	Exposure to cold weather/water Inadequate clothing/shelter Alcohol/drug intoxication Endocrine/metabolic: hypothyroidism, hypoglycaemia, adrenal insufficiency Sepsis, trauma, burns	Remove wet clothing, move to warm environment Passive rewarming: blankets, warm fluids orally Active external rewarming: heating pads, warm air blowers (e.g., Bair Hugger) Severe cases: warmed IV fluids, warmed humidified O₂, peritoneal lavage with warm saline Monitor cardiac rhythm: risk of VF/asystole below 30°C Treat underlying cause

⚖️ Differentials to Consider

Fever (raised hypothalamic set point, responsive to antipyretics) vs hyperthermia (unregulated overheating, not reduced by paracetamol).
Post-ictal hyperthermia vs infective fever.
Hypoglycaemia, intoxication, or stroke mimicking hypothermia clinically.

🎯 Exam Pearls

Heatstroke = medical emergency: CNS dysfunction + high temp. Treat with rapid cooling, not paracetamol.
Malignant hyperthermia (succinylcholine/volatile anaesthetic exposure) → give dantrolene.
Neuroleptic malignant syndrome (antipsychotics) → rigidity + fever → treat with bromocriptine/dantrolene, supportive care.
In hypothermia, don’t declare death until rewarmed to 32°C (“no one is dead until warm and dead”).
ECG in hypothermia: look for J (Osborn) waves.

The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms.

Hyperthermia and Hypothermia

📖 Introduction

🧬 Pathophysiology

🩺 Clinical Features

📊 Comparison Table

⚖️ Differentials to Consider

🎯 Exam Pearls

Categories