Mechanisms of Thermoregulation and Clinical Significance
Our body's normal body temperature is maintained constant by the hypothalamic thermoregulatory center, despite changes in the surrounding environment. This is because the center balances heat production generated by metabolic activities in the muscles and liver with heat dissipation from the skin and lungs.
Fever is defined as an elevation of body temperature that exceeds the normal daily variation, occurring in conjunction with an increase in the hypothalamic set point (e.g., from 37°C to 39°C). The hypothalamus resetting the set point from a normothermic to a febrile level is very similar to resetting a home thermostat to a higher temperature.
Substances that cause fever are called pyrogens. Pyrogens derived from outside the body are called exogenous pyrogens; these include microorganisms (including viruses), microbial products, and microbial toxins. Notably, endotoxin is a substance that readily causes fever in humans.
Fever can be considered a defense mechanism that activates our immune system, reduces the growth rate of bacteria and viruses, and helps our body survive against invading antigens. It is the most basic defense system by which the body protects itself by raising the set temperature.
However, just because fever is a defense mechanism does not mean it always has positive effects. Careful observation is required because hyperpyrexia (high fever >41.5°C), often seen in patients with severe infections or central nervous system (CNS) hemorrhages, or fever associated with severe sepsis or pre-existing cardiopulmonary disease can lead to fatal consequences.
Most fevers are associated with self-limited infections such as the common cold or viral enteritis and rarely progress to high fever. In these cases, depending on the severity of symptoms, one can observe the fever and accompanying symptoms or use antipyretics to expect fever reduction and symptom relief.
When fever symptoms are present, the most important thing is to find the cause. We need to know exactly why the body has activated its defense system to determine whether the current fever is a condition that can be observed or one that requires urgent treatment, and to prepare in advance for any potentially fatal situations.
- Simple Fever: Most fevers are self-limited viral infections and part of the immune response. However, Hyperpyrexia (>41.5°C) or Hyperthermia (due to failure of thermoregulation) must be distinguished and require immediate action.
- Fever and Rash: If a rash appears on the skin along with fever, it is not a simple skin problem. It is an important clue suggesting systemic diseases ranging from infectious diseases like measles and varicella to drug reactions and vasculitis. The cause must be differentiated through the shape and distribution of the rash.
- Fever of Unknown Origin (FUO): If a high fever over 38.3°C persists for more than 3 weeks and no cause is found even after standard tests, persistent follow-up testing is needed, keeping in mind the possibility of hidden infections, autoimmune diseases, or neoplasms.
