Body temperature

The normal human body temperature range is typically stated as 36.5–37.5 °C (97.7–99.5 °F). The average internal temperature is 37.0 °C (98.6 °F). It is typically maintained within this range by thermoregulation. Body temperature normally fluctuates over the day following circadian rhythms, with the lowest levels around 4 a.m. and the highest in the late afternoon.

Core temperature is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system.

Pathophysiology

 * Hyperthyroidism, the overproduction of thyroid hormones, can cause heat intolerance
 * Hypothyroidism, the underproduction or poor utilization of thyroid hormones, can cause cold intolerance

In ME/CFS
Temperature sensitivity, or intolerance to the heat or cold, occurs commonly as a symptom in ME/CFS.


 * 53.9% - 58.7% of the 2073 patients in a Belgian study of 2001 reported symptom exacerbation in extremes of temperature.
 * Katrina Berne reports a prevalence of 75-80% for heat/cold intolerance.

Temperature intolerance is featured as a symptom in the International Consensus Criteria. In the London criteria, sensitivity to both heat and cold is mentioned under the criteria of periods of impaired circulation compatible with autonomic dysfunction.