Should We Reconsider the Definition of Normal Body Temperature?

From Wunderlich’s 37°C to Modern Metrics — Is It Time to Update the Thermometer Standard?

The “Normal” Body Temperature Myth

For over a century, 37°C (98.6°F) has been upheld as the gold standard for “normal” human body temperature. This figure originates from the work of the 19th-century German physician Carl Reinhold August Wunderlich, who based his conclusion on a massive dataset of axillary temperature measurements. But with advances in thermometry, epidemiology, and our understanding of human physiology, many in the medical field now question whether this singular number is still relevant.

Today’s emphasis on personalized medicine demands more than a one-size-fits-all model. Evidence is mounting that average body temperature is decreasing over time, and that interindividual variability is both common and clinically significant. This raises a critical question for modern practitioners: Is it time to redefine what we consider “normal” body temperature?

1. Where Did 37°C Come From?

Wunderlich’s landmark study in 1868 collected millions of temperature readings from over 25,000 patients. Using mercury thermometers placed in the axilla, he calculated the average body temperature as 37°C. He also established the thresholds that defined fever (>38°C) and hypothermia (<36°C).

Despite enormous progress in medical technology and diagnostics, this standard has remained largely unchallenged in clinical practice. Its continued dominance is more a matter of tradition than accuracy.

2. New Data, New Norms

Recent large-scale research efforts—such as the 2020 Stanford study that analyzed data spanning 150 years—have revealed that the average human body temperature is now closer to 36.6°C (97.9°F). This trend appears consistently across:

• Age groups
  
  
• Genders
  
  
• Ethnic backgrounds
  

And importantly, the decline cannot be attributed to errors in measurement or thermometer types. The hypothesized drivers of this shift include:

• A reduced burden of chronic infections, thanks to improved sanitation, antibiotics, and vaccinations
  
  
• Decreased systemic inflammation due to better general health
  
  
• A lower average metabolic rate in modern populations compared to the 19th century
  

This decline, albeit subtle, challenges the ongoing relevance of a fixed temperature standard.

3. Clinical Implications of a New Normal

Clinging to 37°C as the definitive “normal” can have unintended clinical consequences:

• Early signs of infection may be missed in patients whose baseline temperatures are naturally lower
  
  
• Conversely, people with higher natural baselines may be misclassified as febrile
  
  
• In conditions like sepsis, where timely recognition is crucial, a failure to appreciate these variations could delay critical interventions
  
  
• Temperature-based triage systems, especially during pandemics or outbreaks, may be less effective when relying on outdated thresholds
  

The reliance on static values ignores the nuanced reality of human physiology.

4. How Variable Is Body Temperature?

Human body temperature is inherently dynamic, fluctuating within individuals and across populations. Factors influencing this variation include:

• Circadian rhythm: Typically lower in the early morning and higher in the late afternoon
  
  
• Hormonal fluctuations: For example, ovulation causes a measurable rise in basal temperature
  
  
• Environmental influences: Ambient temperature, clothing, and physical activity all contribute
  
  
• Age: Elderly individuals often have lower baseline temperatures
  
  
• Sex: Minor differences between men and women are well-documented
  
  
• Medical status: Medications, thyroid function, and chronic illnesses can alter thermoregulation
  

It’s not uncommon for healthy individuals to have a resting body temperature ranging from 36.1°C to 37.3°C. Yet both values, under current doctrine, are interpreted relative to an arbitrary fixed point.

5. What About Measurement Methods?

Temperature readings vary by the site of measurement and device used:

• Oral measurements tend to be approximately 0.5°C higher than axillary readings
  
  
• Rectal thermometers offer the most consistent internal body temperature but are less practical in many settings
  
  
• Tympanic thermometers provide quick readings but can be influenced by improper technique or earwax
  
  
• Forehead (temporal artery) scanners are widely used for mass screenings but often lack precision
  
  
• Wearable devices are gaining traction and can provide continuous temperature monitoring, though most are still under clinical validation
  

Applying a universal standard like 37°C across all these methods is both inaccurate and potentially misleading.

6. Should We Individualize Temperature Baselines?

One of the most promising approaches involves identifying a personalized “normal” temperature baseline for each patient. Similar to tracking:

• Resting heart rate
  
  
• Baseline blood pressure
  
  
• Weight trajectories
  

This strategy would allow for the detection of clinically significant deviations even in the absence of an absolute fever threshold.

Technology may facilitate this shift. With the proliferation of smartwatches and wearable health trackers equipped with temperature sensors, clinicians and patients can monitor trends over days or weeks. Integration of this data into EMRs could alert healthcare providers to early signs of infection or inflammation before the patient meets traditional febrile criteria.

7. The Febrile Cutoff Dilemma

The current standard defines fever as any temperature exceeding 38°C (100.4°F). However, this one-size-fits-all cutoff fails in various clinical scenarios:

• Elderly patients may present with severe infections without ever reaching 38°C
  
  
• Immunocompromised individuals, including those on chemotherapy or with autoimmune diseases, may show blunted febrile responses
  
  
• Patients with neoplastic or inflammatory conditions may exhibit chronic low-grade temperatures that do not exceed the threshold
  

Relying solely on rigid cutoffs could delay diagnosis or lead to inappropriate reassurance. A contextual and individualized interpretation of temperature is more aligned with modern clinical reasoning.

8. Public Health and Legal Definitions Lag Behind

Temperature screening has become a standard tool in public health, especially during infectious disease outbreaks like COVID-19. Yet, its limitations are increasingly apparent:

• Mass thermal scanners frequently fail to detect febrile individuals
  
  
• Asymptomatic carriers or those with subclinical infections may bypass detection altogether
  
  
• Rigid fever definitions continue to underpin policy decisions, despite growing awareness of variability
  

This disconnect between real-world physiology and institutional standards undermines the reliability of public health measures and can erode public trust.

9. Should Medical Education Update This Standard?

Despite compelling evidence to the contrary, most medical curricula still teach 37°C as the norm. The issue isn’t just about what is taught, but how:

• Medical students often learn to use 38°C as a binary fever cutoff without deeper context
  
  
• Clinical exams reinforce memorization of fixed numbers instead of promoting critical thinking
  
  
• Teaching resources often omit discussions of variability, measurement techniques, and baseline tracking
  

Modernizing medical education to include a range of normal temperatures and an emphasis on individualized patient care is not only appropriate—it is essential. Future clinicians should be equipped to interpret temperature data contextually, not just numerically.

10. Reimagining “Normal”: A Broader Reflection

This debate over temperature mirrors larger shifts occurring in medicine:

• Hypertension management has evolved from universal BP targets to individualized risk-based approaches
  
  
• Lipid guidelines now prioritize cardiovascular risk over isolated LDL numbers
  
  
• BMI is increasingly recognized as a flawed measure when divorced from metabolic health
  

In all of these cases, medicine has transitioned from rigid definitions to a more personalized framework. Body temperature should follow suit. The goal is no longer simply identifying deviation from a number—but understanding what a deviation means for that specific patient.

11. Conclusion: It’s Time to Retire 37°C as Gospel

The idea of a universal “normal” body temperature has persisted more out of habit than scientific rigor. As we shift toward precision medicine and holistic assessment, it's clear that the 37°C standard—while historically important—is no longer sufficient.

Clinicians should:

• Prioritize patient-specific data and temperature trends
  
  
• Understand and educate patients about natural variability in body temperature
  
  
• Use context, rather than absolutes, to guide diagnosis and treatment
  

By retiring outdated standards, we make room for a more accurate, individualized, and nuanced practice of medicine—one that truly aligns with the diverse physiology of our patients.