Normal Body Temperature: Range, Variations & When to Worry

Normal oral temperature: 36.1–37.2°C (97–99°F). Mean 37°C (98.6°F) is a 19th-century average.

Physiological Variations

• Diurnal: Low ~4 AM (36°C/96.8°F), high ~6 PM (37.5°C/99.5°F).
• Age: Infants higher; elderly lower baseline.
• Site: Rectal/tympanic ≈0.3–0.6°C > oral; axillary ≈0.3–0.6°C < oral.
• Activity, ovulation, meals, stress cause fluctuations.

Fever Definitions

• Low-grade: 37.3–38.0°C (99.1–100.4°F)
• Fever: ≥38.0°C (100.4°F) core (rectal/tympanic)
• Hyperpyrexia: >40°C (104°F) – medical emergency

When to Seek Care

• Infant <3 mo: ≥38°C rectal → immediate eval.
• Fever >3 days, >40°C, with lethargy, rash, stiff neck, dehydration, seizure.
• Immunocompromised, recent surgery/travel.