Body Temperature is a measure of the thermal state of the body and reflects the balance between heat production and heat loss. Human temperature is regulated within a relatively narrow physiological range through mechanisms involving the hypothalamus, nervous system, circulation, metabolism and behaviour. There is no single temperature that applies to every healthy person at every moment because values vary with measurement site, time of day, age, activity, hormonal state and individual physiology. Core temperature is more closely related to the temperature of internal organs than surface measurements, while commonly used oral, axillary, tympanic and temporal measurements can differ because they sample different anatomical environments.
The body generates heat primarily through basal metabolism, muscular activity and thermogenesis. Heat is lost through radiation, conduction, convection and evaporation. The hypothalamus coordinates responses that maintain thermal balance, including changes in skin blood flow, sweating and shivering. When the body needs to conserve heat, peripheral vasoconstriction reduces heat transfer to the environment and shivering generates additional metabolic heat. When heat needs to be dissipated, vasodilation increases heat transfer from the skin and sweating enables evaporative cooling. Behavioural responses, such as changing clothing or environmental exposure, are also important components of human thermoregulation.
Fever and hyperthermia are related but distinct phenomena. Fever is a regulated increase in the hypothalamic temperature set point, commonly occurring as part of an immune response to infection or inflammation. Endogenous mediators can alter hypothalamic regulation, producing physiological responses that raise body temperature. Hyperthermia occurs when heat production or environmental heat load exceeds the body's ability to dissipate heat without an intentional increase in the regulatory set point. Heat stroke is a severe form of hyperthermia involving dangerous elevation of body temperature and systemic dysfunction. Treating fever and treating uncontrolled hyperthermia therefore involve different physiological considerations.
Body temperature follows a circadian rhythm, generally reaching a lower level during the early morning and a higher level later in the day. Exercise can temporarily increase temperature because muscular metabolism generates heat, while environmental temperature and clothing can affect measurements and thermoregulation. Hormonal changes can influence temperature regulation, and basal temperature patterns can be used in certain reproductive-health contexts to identify physiological changes associated with ovulation, although this method has limitations. Infants, older adults and people with particular illnesses may have altered thermoregulatory responses, meaning that conventional expectations may not apply uniformly across populations.
Measurement methods differ in accuracy, practicality and clinical suitability. Core temperature can be assessed using invasive or specialised methods in critical-care or research settings, whereas ordinary healthcare commonly uses electronic thermometers at sites such as the mouth, ear or armpit. Rectal temperature is generally closer to core temperature than peripheral measurements but may not be appropriate or necessary in every situation. Tympanic measurements estimate temperature near the tympanic membrane and can be affected by technique and ear conditions. Axillary measurements can be less representative of core temperature. Consequently, temperature readings should be interpreted in relation to the method used rather than assuming that every measurement site is directly interchangeable.
Hypothermia refers to an abnormally low core body temperature, conventionally defined in adults as below 35°C. It can occur after environmental exposure but can also result from impaired thermoregulation, certain medications, endocrine disorders or serious illness. Mild hypothermia may cause shivering and increased sympathetic activity, whereas more severe hypothermia can impair neurological function, cardiac conduction and consciousness and may eventually suppress shivering. Hyperthermia, by contrast, can cause cellular injury, dehydration, neurological dysfunction and organ failure when severe. Both extremes can therefore become medical emergencies depending on severity and circumstances.
Body temperature is an important clinical observation because infection, inflammation, environmental exposure, endocrine disease and other conditions can alter it. However, fever is not synonymous with bacterial infection, and absence of fever does not exclude infection. Some infections occur without significant temperature elevation, particularly in certain older or immunocompromised people. Medications can also influence temperature responses. Clinical interpretation therefore considers symptoms, exposure history, age, immune status, measurement method and other vital signs. A temperature measurement is best understood as one physiological observation within a broader assessment rather than as an independent diagnostic test.
The concept of normal temperature has often been oversimplified into a single numerical value. In reality, physiological temperature varies between individuals and throughout the day, and measurement devices and sites introduce additional variation. Temperature is also different from subjective sensations such as feeling hot or cold. A person can feel chilled while developing a fever because the regulatory set point has changed and the body is generating heat to reach it. Conversely, someone experiencing heat illness may have dangerous hyperthermia even without an infectious process. Understanding these distinctions prevents common errors in interpreting temperature measurements and helps relate them to the underlying mechanisms of thermoregulation.
