Does a Fever Cause Vasoconstriction or Vasodilation? The Complex Answer

October 1, 2025 •

4 min read

The human body's core temperature is tightly regulated, but when an infection strikes, it undergoes a complex process involving both vasoconstriction and vasodilation to create a fever. Contrary to a single outcome, the body's vascular response changes depending on whether the fever is rising or falling.

Quick Summary

A fever involves a two-phase process orchestrated by the hypothalamus: initial vasoconstriction to conserve heat and raise the body's temperature, followed by vasodilation and sweating to release heat as the fever breaks and the body returns to its normal set point.

Key Points

The Dual Response: A fever involves both vasoconstriction and vasodilation, occurring at different phases of the illness.
Rising Fever (Vasoconstriction): As a fever begins, the hypothalamus triggers vasoconstriction, or narrowing of blood vessels in the skin, to conserve heat and raise the body's temperature.
Breaking Fever (Vasodilation): As the fever subsides, the hypothalamus initiates vasodilation, widening the blood vessels to release heat from the body's surface through sweating.
The Hypothalamus Controls the Switch: This entire process is controlled by the hypothalamus, which resets the body's temperature set point in response to infection-fighting chemicals called pyrogens.
Symptoms Reflect the Phase: Chills, shivering, and pale skin are signs of vasoconstriction, while flushed skin and sweating indicate vasodilation and cooling.
Fever is a Defense Mechanism: The raised body temperature can help the immune system function more effectively and inhibit pathogen growth, making the process beneficial in many cases.

The Body's Thermostat: The Hypothalamus

At the core of the body’s temperature regulation is the hypothalamus, a region in the brain that acts as the body's thermostat. Under normal conditions, it maintains a steady internal temperature of around 98.6°F (37°C). When the body is fighting an infection, immune system chemicals called pyrogens are released. These pyrogens travel to the hypothalamus and essentially "reset" the thermostat to a higher temperature. This triggers a series of physiological responses designed to raise the body's temperature to the new, higher set point, which is more hostile to invading pathogens.

Phase 1: The Rising Fever (Vasoconstriction)

In the initial phase of a fever, your body is in heat-conservation mode. Since the hypothalamic set point has been raised, your current body temperature feels too low. To bridge this gap, the body initiates several heat-generating and heat-trapping mechanisms. One of the most noticeable is vasoconstriction, which is the narrowing of blood vessels, especially those in the skin.

During vasoconstriction:

Blood flow is restricted to the skin: The narrowing of superficial blood vessels limits the amount of heat lost to the external environment. Blood is shunted from the extremities and the skin surface towards the core of the body, where the vital organs are located.
You feel cold and shiver: As the blood flow to the skin decreases, the skin may feel pale and cool to the touch, and you may experience chills and goosebumps. Shivering, the rapid contraction and relaxation of muscles, is the body's way of generating additional heat.
The fever increases: These combined efforts of heat conservation and production cause the body's core temperature to rise towards the new, elevated set point.

Phase 2: The Breaking Fever (Vasodilation)

Once the body successfully fights off the infection or fever-reducing medication is administered, the hypothalamus resets the temperature set point back to its normal level. At this stage, the body's core temperature is now higher than the new, lower set point, so the body must begin cooling down. To achieve this, the opposite process, vasodilation, occurs.

During vasodilation:

Blood flow increases to the skin: The blood vessels near the skin surface widen, allowing more blood to flow to the extremities. This increased circulation brings the excess heat from the core to the skin, where it can be dissipated.
Sweating is triggered: The hypothalamus also stimulates the sweat glands to increase perspiration. The evaporation of sweat from the skin provides a highly effective cooling mechanism.
You feel hot and flush: The increased blood flow to the skin often causes a flushed appearance and a sensation of warmth. It is a sign that the body is successfully cooling down and the fever is subsiding.

Understanding the Distinction: Fever Vasoconstriction vs. Vasodilation


Characteristic	Rising Fever (Vasoconstriction)	Breaking Fever (Vasodilation)
Hypothalamic Set Point	Elevated	Reset to normal
Blood Flow to Skin	Decreased	Increased
Body Sensation	Chills, feeling cold	Feeling warm, flushed skin
Purpose	Conserve heat to raise body temperature	Dissipate heat to lower body temperature
Physical Signs	Pale skin, goosebumps	Flushed skin, sweating

Why This Process is Beneficial

Far from being a dangerous malfunction, a fever is an evolutionarily conserved defense mechanism that serves several purposes. The increased temperature can help inhibit the growth of certain pathogens that thrive at normal body temperatures. Additionally, fever has been shown to enhance various immune cell functions, making the immune system more effective at fighting off infection. The entire process, from the initial vasoconstriction to the final vasodilation, is a carefully orchestrated response by the immune system to regain health.

When to Seek Medical Attention

While most fevers are a natural and temporary part of fighting illness, it is important to know when to seek professional medical advice. For children under 3 months, any fever of 100.4°F (38°C) or higher warrants immediate medical evaluation. For older children and adults, a fever is generally not dangerous unless it reaches very high levels or is accompanied by other severe symptoms.

Seek immediate medical attention if a fever is accompanied by:

Stiff neck or severe headache
Confusion, extreme irritability, or lethargy
Difficulty breathing or chest pain
A purplish or spotted rash
A seizure

Conclusion

In summary, a fever is a dynamic, multi-stage process that involves both vasoconstriction and vasodilation, not one or the other. During the initial, ascending phase, the body actively conserves heat through vasoconstriction, causing you to feel cold and shiver despite your rising temperature. Once the infection is being cleared and the body needs to cool down, the blood vessels dilate, and you begin to sweat as the fever breaks. Understanding this thermoregulatory process highlights the body's sophisticated defense system and helps explain the different sensations experienced during a fever.

Learn more about what happens during a fever from the Cleveland Clinic: What Happens When Your Body Has a Fever?

Frequently Asked Questions

A fever causes both, but at different times. It first causes vasoconstriction to raise the body's temperature, then triggers vasodilation to lower it once the fever has broken.

The chills and shivering that occur as a fever starts are caused by vasoconstriction. The hypothalamus resets the body's temperature higher, and the body feels cold by comparison, so it shivers to generate heat.

Sweating is a cooling mechanism. When your fever breaks, your body's temperature set point returns to normal, and it must release excess heat. The hypothalamus initiates vasodilation and sweating to facilitate this cooling.

The hypothalamus is the body's thermostat. When an infection is present, chemicals called pyrogens elevate its set point, initiating the fever process. The hypothalamus then orchestrates vasoconstriction and vasodilation to regulate temperature according to this set point.

Fever-reducing medications like acetaminophen and ibuprofen work by lowering the hypothalamic set point. While they can improve comfort, some studies suggest that in many infections, allowing a fever to run its course may help the immune system. It is often best to focus on comfort rather than normalizing temperature completely, unless medically advised.

Fever is a regulated and controlled rise in body temperature in response to an illness, managed by the hypothalamus. Hyperthermia is an uncontrolled rise in body temperature caused by external factors (like heatstroke), and the body's thermoregulation mechanisms cannot cope.

Seek medical attention if a fever is very high (e.g., above 103°F or 39.4°C in adults) or is accompanied by symptoms like a severe headache, stiff neck, confusion, breathing difficulties, or a seizure. Always consult a doctor for a fever in an infant under 3 months.