Thermoneutrality

Thermoneutrality is the physiological state in which an endothermic organism can maintain a stable core body temperature with minimal metabolic energy, achieved within a specific ambient temperature range known as the thermoneutral zone (TNZ). Within the TNZ, heat loss and heat production balance through sensible heat exchange with the environment, and the resting metabolic rate remains at its minimum for thermoregulation.

The TNZ is bounded by a lower critical temperature (LCT) and an upper critical temperature (UCT). Below the LCT, the body must increase metabolic heat production to prevent hypothermia; above the UCT, it must dissipate excess heat. In humans, estimates of the TNZ at rest with minimal clothing lie roughly in the mid-20s to around 30°C, commonly cited as about 25–27°C. Actual values vary with clothing insulation, humidity, air movement, activity, body composition, age, acclimatization, and gender.

Physiological responses outside the TNZ include shivering and non-shivering thermogenesis in cold conditions; sweating and evaporative cooling in heat; vasomotor adjustments also help regulate heat loss. The contribution of brown adipose tissue to non-shivering thermogenesis is more prominent in some species and in humans is typically limited to infancy or cold exposure.

The concept is used in physiology, environmental design, and clinical contexts (for example, anesthesia and fever research). It is species-specific and size-dependent; smaller animals generally have higher metabolic rates and often narrower TNZs, while larger animals tend to have broader ranges. The TNZ applies to endotherms; ectotherms do not share this zone in the same sense.