Hyperthermia: Fever vs External Heat

The critical differences in mechanisms, and why you should not use a sauna when you have a fever 

Both fever and external heat exposure can raise your core body temperature to similar degrees. But they represent fundamentally different physiological mechanisms.  In fact, parts of the processes could be said to be opposites. Which is intriguing and curious, and also has important safety implications. 

First, what is “core temperature” and why does it matter so much?

Human core body temperature is the temperature of the vital organs within the thoracic and abdominal cavities.  It is maintained within a narrow range, approximately 36º-38ºC (96.8–100.4°F), under normal conditions. This precise control reflects the narrow range at which our cells and particularly our proteins are designed to function most efficiently. 

Core body temperature and protein shape

Proteins are the molecules in our cells that “do” everything. They carry out all our cellular processes. They catalyze reactions, control movement of fluids and nutrients, transport substrates, form the structure of the cell, and more. A protein’s shape is essential to its function. Shape must have some degree of fluidity,  as all these processes are dynamic and many require slight shifts in shape as they do their jobs.

Between 36–38°C our proteins are at the ideal flexibility to carry out their functions. But with more heat, they become unstable, and with too much heat they start to unfold and become dysfunctional. When many proteins become unfolded, they start to clump together, forming protein aggregates that are toxic to the cell. We want to avoid this.  

Thermoregulation and the thermostat in your brain

The hypothalamus is a deep brain structure just above the midbrain that is responsible for maintaining temperature homeostasis.  It receives information from temperature sensors throughout the body, like tiny thermometers set up in our core and vital organs, as well as our skin and peripheral muscles.  If temperature starts to drift from its “set point” (normally 98.6ºF), it initiates mechanisms to compensate. The body has mechanisms to dissipate heat when temperatures rise (sweating, decreased activity), and others to generate and conserve heat when our temperature drops (shivering, decreasing surface area by huddling, increasing metabolism).

Heat dissipation: how your brain cools you down

When our core temperature starts rising, or the hypothalamus determines we are under heat stress (it can learn to detect a recurring impending heat stress before our core temp rises at all), it increases blood flow to the skin, and activates eccrine sweat glands. These are our two primary mechanisms of heat dissipation, and they work together. 

Our extremities are normally several degrees cooler than is our core, and this gradient can increase depending on conditions and source of heat. Blood that flows through the skin cools, and the cooler blood is then brought back to the core. This works together with the activation of eccrine sweat glands in the skin which is our most important cooling mechanism.  Sweat evaporation can continue to cool the skin and blood even when air temperature is higher than the body. 

Fever: our thermostat Reset

when your fever is rising, your hypothalamus thinks you are hypothermic.  If the set point is 102ºF, anything below that, you will feel cold and be shivering under blankets.  And this is why when the fever “breaks,” and the set point drops back to normal, suddenly you are throwing the 10 blankets off and sweating profusely.  

Fever: The thermostat turned up

Clinically, a fever is defined as a core temperature of ≥38°C (100.4°F). With a fever, the body’s thermostat in the  hypothalamus is set higher than normal.  This is usually in response to an infection, where the immune system is active and releasing pyrogens, which cause this bump in the internal thermostat. The body actively generates heat through shivering, vasoconstriction, and behavioral responses such as huddling up to reduce surface area exposure, and seeking warmth. 

This is why, when your fever is rising, regardless of what your actual body temperature is, your hypothalamus thinks you are hypothermic.  If the set point is trying to get you to 102ºF, anything below that, 101.5ºF even, you will feel cold and be shivering under blankets.  And this is why when the fever “breaks,” and the set point drops back to normal, suddenly you are throwing the 10 blankets off and sweating profusely.  

Importantly, thermoregulatory control is preserved. Even when temperatures exceed 40°C (104°F), fever itself rarely causes direct harm because the hypothalamus continues to regulate the process.

External heating: Body temperature rises despite the thermostat’s cooling efforts

I am going to talk about hyperthermia in the context of thermal therapy, where one intentionally raises their core body temperature through modalities such as a sauna or hot tub.    During thermal therapy, core temperatures can reach 38–39°C (100.4–102.2°F). This is generally the goal. It is a slight stress on the body that results in strengthening our cellular responses, and activates heat shock proteins.  What is important, and why it very rarely results in heat illness or heat stroke, is that we remain in control, able to remove the heat source and lower our temperature when we are ready

Hyperthermia occurs when a heat source is putting heat into the body, or preventing the body from dissipating heat, and the body cannot maintain temperature homeostasis.  The body's cooling mechanisms are in full force, but they are being overwhelmed by the external heat. 

The set point of the “thermostat” that the hypothalamus is aiming for, has not changed.  It is trying to get you to 98.6ºF.  So, compared with our rising fever example where we had maximal heat production and conservation mechanisms at 101ºF, in a sauna, or with external heating, you will be sweating and feel hot.  The hypothalamus has activated maximal cooling responses: profuse sweating, cutaneous vasodilation, and increased cardiac and respiratory output.  They are not keeping up, and your body temperature is still rising, but it is important to note here that they are having an effect, and without them your body temperature would be much higher,   

Why avoid sauna use with a fever

In healthy individuals with intact thermoregulatory function, controlled heat exposure poses minimal risk, as the body’s cooling mechanisms remain active throughout the session.  While body temperature is elevated, our cooling mechanisms keep it from becoming dangerously elevated.  

Thermal therapy is not recommended during active fever, as the elevated hypothalamic set point and altered thermoregulatory responses make heat exposure unpredictable. The body is not trying to keep the body temp at our normal 98.6º.  It may be set at 104ºF, so with an added heat source, our temp would go straight there before cooling mechanisms would even start.  We don’t know where it would land. 

We do know that your thermoregulatory mechanisms are not functioning normally when you have a fever, and unlike when you are healthy, we do not know how your body will respond to the heat.  So it is safest to avoid heat therapy while sick with a febrile illness.