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 (body clock), 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
The following are diseases and disorders in which issues in thermoregulation may occur. Each may present in people with ME or ME/CFS.
 * Small fiber neuropathy, a type of neuropathy due to damage to the small myelinated and unmyelinated peripheral nerve fibers present in skin, peripheral nerves and organs
 * Neurological diseases and disorders such as Parkinson's or multiple sclerosis
 * Damage to the central nervous system or spinal cord, particularly those above T6 can cause dysregulation; “noxious stimuli” will trigger or worsen symptoms
 * Hyperthyroidism, the overproduction of thyroid hormones, can cause heat intolerance
 * Hypothyroidism, the underproduction or poor utilization of thyroid hormones, can cause cold intolerance
 * Iron deficiency anemia, due to:
 * iron's effect on thyroid function
 * competing demands re: oxygenation versus decreases in blood flow to mitigate heat loss
 * iron's status as a cofactor in many of the enzymes governing metabolic rate

Temperature dysregulation
Temperature dysregulation, heat intolerance and cold intolerance are common symptoms of ME/CFS, and are is recognised in the Canadian Consensus Criteria as a neuroendocrine symptom.


 * 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.

In the London criteria for ME, sensitivity to both heat and cold is mentioned under the criteria of periods of impaired circulation compatible with autonomic dysfunction.

Temperature instability
Temperature instability and heat or cold intolerance are ME symptoms recognized in the International Consensus Criteria.

Loss of thermostatic stability and intolerance of extremes of temperatures are separate diagnostic criteria in the International Consensus Criteria for ME. Indicators of temperature instability are listed as subnormal body temperature, marked fluctuations of temperature throughout the day; sweating episodes, recurrent feelings of feverishness with or without low grade fever, and cold extremities, e.g. fingers and toes.