Understanding the body's thermostat and fever
Your body's internal temperature is regulated by the hypothalamus, a region in the brain that acts as the body's thermostat. When your immune system detects an infection, it releases chemicals called pyrogens that signal the hypothalamus to raise the body's set point temperature. This controlled increase, known as a fever, is a natural defense mechanism intended to create a less hospitable environment for viruses and bacteria. It is a sign that your body is actively fighting an illness, not an illness in itself.
Trigger temperatures by measurement method
For practical clinical purposes, the temperature that signifies a fever varies based on where the measurement is taken. This is because temperatures naturally differ throughout the body.
Rectal (most accurate for young children)
- Trigger temperature: 100.4°F (38°C) or higher.
- When to use: Often recommended for infants and young children under 3 years old, as it provides the most accurate core body temperature reading.
Oral (mouth)
- Trigger temperature: 100°F (37.8°C) or higher.
- When to use: A common method for individuals over the age of 4 who can hold a thermometer properly under their tongue.
Tympanic (ear)
- Trigger temperature: 100.4°F (38°C) or higher.
- When to use: Measures infrared heat from the eardrum. Can be inaccurate if not placed correctly or if there is too much earwax. Not recommended for infants under 6 months old.
Temporal artery (forehead)
- Trigger temperature: 100.4°F (38°C) or higher.
- When to use: A non-invasive method using an infrared scanner across the forehead. Considered a quick and convenient option, but accuracy can sometimes be affected by outside temperature or improper use.
Axillary (armpit)
- Trigger temperature: 99°F (37.2°C) or higher.
- When to use: The least accurate method for determining a fever but can be used for initial screening. If the armpit temperature is elevated, it should be confirmed with a more accurate method.
Different considerations for adults and children
Age is a significant factor in determining when a fever is a cause for concern. While a low-grade fever in a healthy adult may not require medication, a fever in a young infant is a serious medical concern.
Adults
- General guideline: A fever of 100.4°F (38°C) or higher.
- When to see a doctor: Seek medical attention if a fever reaches 103°F (39.4°C) or higher, lasts for more than 48–72 hours, or is accompanied by severe symptoms like confusion, a stiff neck, or difficulty breathing.
Infants and young children
- Infants under 3 months: A rectal temperature of 100.4°F (38°C) or higher is considered a medical emergency. Contact a healthcare provider immediately.
- Children 3 to 6 months: Contact a healthcare provider for a rectal temperature higher than 102°F (38.9°C), or for any fever if they seem unusually irritable or sluggish.
- Children 7 to 24 months: Consult a doctor if a rectal temperature higher than 102°F (38.9°C) lasts for more than one day, especially if other symptoms are present.
Comparison of fever temperatures by measurement type
To help visualize the differences, the following table compares the fever threshold and typical accuracy for the most common temperature measurement types.
| Measurement Method | Fever Threshold | Accuracy | Common Use | Notes |
|---|---|---|---|---|
| Rectal | 100.4°F (38°C) | High | Infants and young children | Most reliable for core body temp |
| Oral (Mouth) | 100°F (37.8°C) | Medium | Adults and older children | Wait 30 mins after hot/cold drinks |
| Tympanic (Ear) | 100.4°F (38°C) | Medium-High | Children over 6 months and adults | Placement must be correct for accuracy |
| Temporal Artery (Forehead) | 100.4°F (38°C) | Medium | All ages | Convenient but less precise than rectal |
| Axillary (Armpit) | 99°F (37.2°C) | Low | Screening purposes | Least accurate, should be confirmed |
Other causes of elevated body temperature
It's important to remember that not all elevated body temperatures are caused by illness. Non-infection related causes can also result in a raised temperature. These include:
- Intense physical activity: Vigorous exercise can temporarily increase body temperature.
- Heat exposure: Heat exhaustion or heatstroke can cause dangerous elevations in body temperature.
- Certain medications: Some antibiotics and other drugs can cause a fever as a side effect.
- Immunizations: A mild fever is a common and normal reaction to some vaccines.
- Autoimmune conditions: Diseases like rheumatoid arthritis can cause fevers as a result of systemic inflammation.
Conclusion
Understanding what is the trigger temperature for fever is the first step in managing an illness, but it is not the only piece of the puzzle. The definition of a fever depends on the measurement method, and what constitutes a serious fever varies with age and other symptoms. While most fevers in adults are not dangerous and can be managed at home with rest and fluids, any fever in a newborn is a medical emergency. For specific guidance on fever management and when to seek medical help, particularly concerning a child's fever, consult reputable medical resources like the American Academy of Pediatrics, or your healthcare provider directly. Monitoring for concerning symptoms like confusion, a stiff neck, or difficulty breathing is always a priority, regardless of the temperature reading itself.
Link to American Academy of Pediatrics guidance for further reading on fevers in children.
