Body Temperature: Fever, Hyperpyrexia vs Hyperthermia (heat stroke)

The hypothalamus controls body temperature. A human’s core body temperature is 36.5 – 37.5 degree Celsius Or 97.7 to 99.5 Fahrenheit.
Core body temperature means, the temperature in internal organs. Axillary temperature is around 0.6 degrees lower than core body temperature.
It is difficult to define the exact number what is normal body temperature. But for 95 % of a healthy adult population, normal oral temperature variates from 35.7 – 37.3 degree Celsius. And the mean temperature was 36.6 degrees Celsius. Axillary temperature can be lower.
So, an oral temperature, of more than 37.7 degree Celsius, or 99.9 Fahrenheit can be defined as fever.
Body temperature has diurnal and seasonal variations.
Body Temperature is lower in summer and higher in winter.
Lowest in the early morning and highest in the early evening.
Core body temperature slightly lowers with age. And increases with comorbid conditions, like cancer and some chronic diseases.
Rectal temperature is generally higher, than oral temperature. The difference is around 0.4 degrees.
In reproductive age women. early morning temperature is lower before ovulation. After ovulation degree increases around 0.6 degrees Celsius or 1 Fahrenheit, and stays increased until menstruation occurs.
Fever - Defined as higher body temperature, than normal. But still controllable by the hypothalamus.
Mechanism of Fever is:
First. Peripheral vasoconstriction prevents heat loss and increases internal organs’ temperature.
The Second mechanism is shivering, which constriction of muscles and increases heat production.
And third. When a person feels cold, putting on more clothing can further prevent heat loss.
Another mechanism can be liver heat production. The liver is metabolically active and it always produces heat that is dissipated, but vasoconstriction prevents heat loss, and body temperature increases sometimes without even shivering.
When a person takes antipyretic medication, the hypothalamus also regulates the downward process.
Sweating and vasodilatation increase heat loss, also behavioral changes, and the removal of clothes, facilitate the heat loss process.
Fever, More Than 41.5 degree Celsius, or 106 Fahrenheit, is called hyperpyrexia.
Hyperpyrexia occurs rarely and can be associated with very severe infections. Or rarely hypothalamic trauma, cancer, or intrinsic hypothalamic malfunction.
If temperature increases uncontrollably, where the body’s defense mechanisms are unable to control temperature, this is not called fever anymore. It is called hyperthermia.
Hyperthermia is also called heat stroke.
The exogenous heat environment is the most important factor to develop hyperthermia.
For example strenuous labor when outside is too much hot. Or work out in a hot environment.
In this case happens both, internal heat production and environmental hot exposure. In this case, body is unable to compensate for such heat exposure and heat loss is not enough.
In the case of hyperthermia, pyrogenic molecules are not involved in heat production.

Hyperthermia can easily be fatal. And it is always an emergency condition.
Antipyretics aren’t effective in case of hyperthermia, while in case of fever, even if it is hyperpyrexia they are effective at least to some degree.
Management of Hyperthermia:
The first step is to remove the underlying cause. Mild hyperthermia caused by exertion on a hot day may be adequately treated through increased water consumption and resting in a cool place.
But severe hyperthermia needs urgent treatment.
Passive cooling techniques, such as resting in a cool, shady area and removing clothing can be applied immediately.
Drinking water and turning a fan or dehumidifying an air conditioning unit on the affected person may improve the effectiveness.
Sitting in a bathtub of tepid or cool water, can remove a significant amount of heat.