Feeling cold while running a fever is a common experience, but it is a normal part of the body’s defense mechanism. A fever is a temporary, regulated increase in the body’s internal temperature set point. This set point is typically regulated around 98.6°F (37°C). A fever occurs when this internal thermostat is intentionally reset higher, usually to 100.4°F (38°C) or more. The sensation of coldness and subsequent shivering result from the body working hard to reach this newly elevated target temperature.
The Hypothalamus and Temperature Regulation
The body’s central temperature control center resides in the hypothalamus, a small region of the brain that functions much like a home thermostat. This area constantly monitors the body’s core temperature, ensuring it remains within a narrow, healthy range. When an infection occurs, the immune system releases chemical messengers called pyrogens into the bloodstream.
These pyrogens travel to the brain and act on the hypothalamus, initiating the fever response. They trigger the production of Prostaglandin E2 (PGE2), which resets the hypothalamic thermostat to a higher temperature, perhaps from 98.6°F to 102°F. Since the body’s actual temperature is below this new set point, the body feels cold.
The hypothalamus interprets the current body temperature as a deficit against the new, elevated target. To close this gap, the brain activates mechanisms designed to generate and conserve heat. This response aims to warm the body up to meet the new temperature the hypothalamus considers normal. This controlled increase in temperature helps the immune system fight off pathogens that do not thrive in warmer conditions.
Understanding Chills and Shivering
The feeling of being cold, known as chills, is the subjective sensation that prompts the body to seek warmth. This feeling is accompanied by physical mechanisms that increase heat production. One primary response is peripheral vasoconstriction, where blood vessels near the skin’s surface narrow to reduce heat loss.
The noticeable physical reaction is shivering, which involves rapid, involuntary contractions of skeletal muscles. Shivering is an effective mechanism for heat generation because muscle movement converts stored chemical energy directly into thermal energy. This muscular activity is a direct instruction from the hypothalamus to raise the core temperature toward the new set point.
The severity of chills and shivering often correlates with the speed at which the fever is rising. Once the body’s temperature reaches the new, higher set point, the chills typically subside. Conversely, when the fever “breaks” and the set point returns to normal, the body initiates cooling mechanisms like sweating and vasodilation to lower the temperature.
When to Seek Medical Attention
While feeling cold with a fever is a normal physical process, certain signs indicate the illness may be serious and require prompt medical attention. For adults, a fever reaching 103°F (39.4°C) or higher should cause concern, especially if it does not respond to medication. If an adult fever lasts more than 48 hours without improvement, consult a healthcare provider.
Immediate medical attention is necessary if the fever and chills are accompanied by other severe symptoms. These warning signs warrant an immediate visit to the emergency room:
- A severe headache, a stiff neck, or an unusual sensitivity to bright light.
- Any change in mental status, such as confusion, difficulty waking up, or altered speech.
- Difficulty breathing or chest pain.
- A rash that spreads rapidly or appears as small, purplish spots.