Subfebrile temperature

Subfebrile temperature: Hidden signal or normal?

Subfebrile temperature (subfebrility) is a persistent elevation of body temperature in the range of 37.1°C to 38.0°C. Unlike high fever, which is an acute response to infection, subfebrility is often prolonged, exhausting, and may not be accompanied by other striking symptoms. It presents a diagnostic challenge for physicians because the causes can be very diverse: from completely harmless physiological fluctuations to signs of serious chronic diseases.

When is subfebrility normal?

Not every temperature rise to 37.2°C indicates illness. There are physiological reasons:

• Diurnal variations: In the evening body temperature is always slightly higher than in the morning.
• Menstrual cycle phase: In women during the second phase of the cycle (post‑ovulation) temperature rises by 0.3‑0.5°C.
• After physical exertion or stress.
• In infants: Due to immature thermoregulation, as discussed in the article "Subfebrile temperature in infants".

Subfebrility as a symptom of disease

Persistent subfebrility lasting weeks or months can be a signal of a “smoldering” inflammatory process in the body.

• Chronic infections: It may indicate a focus of chronic infection (e.g., tonsillitis, sinusitis, urinary tract infections) or more serious conditions such as tuberculosis.
• Post‑viral “temperature tail”: After an upper respiratory infection, subfebrile temperature can persist for another 2‑4 weeks while the nervous system restores thermoregulation.
• Autoimmune diseases: Systemic connective‑tissue diseases (rheumatoid arthritis, lupus) are often accompanied by subfebrility.
• Endocrine disorders: For example, hyperthyroidism (overactive thyroid gland).

If subfebrility lasts a long time and is accompanied by fatigue and malaise, it warrants thorough examination. Sometimes, when the cause cannot be identified, the condition is classified as fever of unknown origin.