The human body maintains a remarkably stable internal temperature, a process known as thermoregulation, which is handled by the hypothalamus in the brain. This control is necessary because the body’s numerous chemical reactions and organ functions are designed to operate within a very narrow thermal window. The accepted normal range for core body temperature, or normothermia, is typically stated to be between 97.7°F (36.5°C) and 99.5°F (37.5°C), though this can vary slightly throughout the day and between individuals. When the body’s internal temperature strays outside of this narrow range, it signals an abnormality that requires attention. Understanding the mechanisms behind these temperature deviations is important for recognizing the severity of the condition.
Fever and the Immune Response
Fever, or pyrexia, is a controlled elevation of the body’s temperature that occurs when the thermoregulatory set point in the hypothalamus is intentionally raised. This increase is usually triggered by pyrogens, which are fever-inducing substances released by the immune system in response to infection, inflammation, or certain non-infectious causes like malignancy. The body perceives its previous normal temperature as too cold and initiates mechanisms like shivering and vasoconstriction to generate and conserve heat until the new, higher set point is reached. A temperature at or over 100.4°F (38°C) is generally considered a fever.
The elevated temperature acts as an alert system, activating immune surveillance. The hypothalamus creates a systemic response to match the new, higher temperature. Once the threat is neutralized and the pyrogens dissipate, the hypothalamus resets the temperature back to normal, leading to heat loss mechanisms like sweating and flushed skin.
Hyperthermia and Heat-Related Illnesses
Hyperthermia is distinct from a fever because it represents an uncontrolled rise in core body temperature where the hypothalamic set point remains normal. This condition occurs when the body’s ability to dissipate heat is overwhelmed by an excessive external heat load or internal heat production. Causes include strenuous physical activity in hot weather or high ambient temperatures.
Severe hyperthermia, such as heatstroke, is a medical emergency where the core temperature can exceed 104°F (40°C) and can quickly lead to organ damage. Symptoms include hot, dry skin due to the cessation of sweating, confusion, nausea, and, in severe cases, impaired consciousness or convulsions. Since the thermoregulatory set point is not elevated, fever-reducing medication is ineffective. The heat buildup can result in consequences including chronic neurological damage and organ failure, emphasizing the need for rapid cooling.
Hypothermia and Core Temperature Drop
Hypothermia is defined as a dangerously low core body temperature, typically below 95°F (35°C), resulting from the body losing heat faster than it can generate it. Common causes include prolonged exposure to cold environments or immersion in cold water. When the temperature drops, the heart, nervous system, and other organs begin to malfunction.
The initial phase, or mild hypothermia (90°F–95°F or 32°C–35°C), is marked by intense shivering and a decline in memory and judgment. As the condition progresses to moderate hypothermia (82°F–90°F or 28°C–32°C), shivering often ceases, and confusion increases, sometimes leading to slurred speech and loss of coordination. In severe hypothermia, the person may become unresponsive. A notable symptom is paradoxical undressing, where the confused individual removes clothing despite the cold. This progression can lead to respiratory and cardiac failure if not addressed quickly.
Detecting and Addressing Severe Temperature Changes
Accurate measurement is the first step in detecting a temperature abnormality, but reliability depends heavily on the method and site used. Rectal and esophageal measurements are the most accurate reflection of true core body temperature, though they are invasive and typically used in clinical settings. For home use, digital thermometers measuring oral or tympanic (ear) temperatures are practical alternatives.
Any temperature abnormality requires prompt medical evaluation, especially when accompanied by severe symptoms. For elevated temperatures, immediate medical attention is necessary if a fever exceeds 104°F (40°C) or if the person shows signs of confusion, seizures, or loss of consciousness, which can indicate heatstroke or hyperpyrexia. When dealing with high temperatures, basic first aid involves physical cooling, such as removing excess clothing, fanning the person, and applying cool packs to areas like the armpits and groin while waiting for professional help. For low temperatures, hypothermia should be suspected if the core temperature is below 95°F (35°C) or if the person is shivering uncontrollably, is confused, or has a weak pulse. Initial warming steps include moving the individual to a warm, dry location, removing wet clothing, and covering them with blankets to prevent further heat loss.