The nervous system is the primary body system that regulates body temperature, with a small region of the brain called the hypothalamus acting as your internal thermostat. But temperature regulation isn’t a solo act. Your skin, circulatory system, muscular system, and endocrine system all work together under the hypothalamus’s direction to keep your core temperature within a narrow range, generally between 97°F (36.1°C) and 99°F (37.2°C).
The Hypothalamus: Your Brain’s Thermostat
The hypothalamus sits near the base of your brain and contains specialized neurons that continuously monitor your blood temperature. A cluster of these neurons in an area called the preoptic area (POA) is especially important. About 20 to 40 percent of neurons in this region are “warm-sensitive,” meaning they detect when blood flowing through the brain gets warmer and trigger cooling responses. A neighboring region called the dorsomedial hypothalamus contains neurons that activate when you’re cold.
The way these two groups interact is elegant. When your body heats up, the warm-sensitive neurons in the preoptic area fire signals that directly suppress the cold-activated neurons. This lowers your core temperature by dialing down heat production and ramping up heat loss. When you’re cold, the process reverses: the cold-activated neurons take over, triggering shivering, goosebumps, and blood vessel constriction to conserve warmth.
The hypothalamus doesn’t rely on brain temperature alone. It also receives input from temperature sensors embedded throughout your skin, giving it a real-time picture of your environment before your core temperature has even changed. This lets your body respond proactively, starting to sweat before you overheat or constricting blood vessels before you lose too much warmth.
How Your Skin Releases Heat
Your skin is the body’s largest organ and the primary surface where heat actually leaves the body. It does this through four mechanisms: radiation (heat radiating off exposed skin), convection (air or water carrying heat away), conduction (direct contact with cooler surfaces), and evaporation of sweat.
Evaporative cooling through sweat is the most powerful of these, especially when you’re physically active or in hot conditions. Your body has roughly 125 to 200 eccrine sweat glands per square centimeter of skin, though the density varies by body region. When the hypothalamus signals that you’re too warm, the autonomic nervous system activates these glands, which secrete sweat onto the skin surface. As that moisture evaporates, it pulls heat energy away from the skin. In dry air, this process is highly efficient. In humid conditions, sweat doesn’t evaporate as easily, which is why muggy heat feels so much more oppressive.
Your body increases sweat output in two ways simultaneously: recruiting more individual glands and increasing how much each active gland produces. With repeated heat exposure over about a week, your sweating system actually recalibrates. The temperature at which sweating kicks in drops lower, and the overall output increases. This is heat acclimatization, and it’s why the first hot day of summer feels brutal compared to the tenth.
Blood Flow as a Heat Shuttle
Your circulatory system acts as an internal heat-distribution network. Blood carries warmth generated deep in your organs and muscles, and the nervous system controls how much of that warm blood reaches the skin surface.
In hot conditions, blood vessels near the skin widen (vasodilation), allowing a large percentage of cardiac output to flow close to the surface where heat can escape into the surrounding air. A specialized sympathetic vasodilator system is responsible for 80 to 90 percent of this skin-directed blood flow increase during heat stress. In cold conditions, the opposite happens. Blood vessels near the skin constrict (vasoconstriction), keeping warm blood deeper in the body and reducing heat loss from the surface. This is why your fingers and toes get cold first: blood is being redirected away from your extremities to protect your vital organs.
This system has a trade-off. During intense heat, so much blood is directed toward the skin that your cardiovascular system has to work harder to maintain blood pressure. That’s one reason people with heart conditions are more vulnerable to heat-related illness.
Muscles Generate Emergency Heat
When you’re cold, the hypothalamus triggers shivering, which is rapid, involuntary contraction of skeletal muscles. These contractions don’t produce useful movement, but they generate significant heat. Shivering can increase your body’s heat production to as much as five times your resting metabolic rate, a substantial boost that can maintain core temperature even in cold environments.
Shivering is an energy-intensive process, which is why prolonged cold exposure leaves you feeling exhausted and hungry. Your body burns through glucose and fat stores rapidly to fuel these contractions, and the hypothalamus coordinates with the endocrine system to keep those fuel supplies available.
Hormones Set Your Baseline Heat Output
While the nervous system handles moment-to-moment temperature adjustments, the endocrine system shapes your baseline heat production over longer time periods. Thyroid hormones play the central role here. These hormones regulate your basal metabolic rate, essentially how much energy your cells burn at rest, and a byproduct of that energy use is heat.
When you’re exposed to cold over days or weeks, your brain’s hypothalamic-pituitary-thyroid axis ramps up, increasing thyroid hormone levels. This boosts glucose production and delivery to metabolically active tissues like liver and muscle, providing the fuel needed to sustain heat production during prolonged cold. People with an underactive thyroid often feel cold all the time because their baseline heat production is lower than normal. Those with an overactive thyroid may feel warm and sweat excessively because their metabolic furnace runs too hot.
Brown Fat: A Hidden Heat Source
Beyond shivering, your body has another way to generate heat without any muscle movement. Brown adipose tissue, commonly called brown fat, contains cells packed with mitochondria that can burn calories purely to produce warmth. This process, called non-shivering thermogenesis, depends on a protein called UCP1 that essentially short-circuits the normal energy-production pathway, converting fuel directly into heat instead of usable cellular energy.
Brown fat is activated by the same branch of the nervous system that controls shivering, using the chemical messenger norepinephrine. In animal studies, all cold-adapted non-shivering heat production has been traced back to brown fat and UCP1 specifically. Adults were long thought to have very little functional brown fat, but imaging studies have since confirmed that most adults retain some, particularly around the neck and upper back. Its contribution to overall temperature regulation in adults is likely smaller than in infants (who can’t shiver effectively), but it does play a role, especially during gradual cold adaptation.
How Fever Overrides the System
Fever is not a malfunction of thermoregulation. It’s a deliberate reset. When your immune system detects an infection, immune cells release signaling molecules called pyrogens. These trigger the production of a chemical messenger, prostaglandin E2, at the boundary between your bloodstream and brain. Prostaglandin E2 then raises the hypothalamus’s temperature set point from its normal level of around 97 to 99°F up to a higher target.
Once the set point is elevated, your body “thinks” it’s too cold at its current normal temperature. It responds the same way it would in a cold environment: blood vessels constrict (which is why you may feel chilled and pale at the start of a fever), muscles may shiver, and metabolic heat production increases. These responses continue until your blood temperature climbs to match the new, higher set point. A temperature above 100.4°F (38°C) generally indicates a fever. When the infection resolves and pyrogen levels drop, the set point returns to normal, and you suddenly feel hot, beginning to sweat and flush as your body sheds the excess heat.
Common fever-reducing medications work by blocking the production of prostaglandin E2, which lowers the set point back toward normal rather than directly cooling the body.