Cortisol

Cortisol is a steroid hormone released by the adrenal glands which sit upon that kidneys. Release of cortisol is controlled by the hypothalamus, a part of the brain. Cortisol has several known roles in the human body: aiding in metabolism, reducing inflammation, assisting electrolyte balance, stimulating gastric-acid secretion and controlling one's sleep/wake cycle.

Normal blood level of cortisol vary throughout the day, with the highest level in the morning and lowest level around midnight. Levels can also rise in reaction to stress or low blood sugar. Some health conditions can cause an increase in cortisol, other health conditions can cause a decrease.

ME/CFS
Hypocortisolism (low cortisol levels) is common in patients with ME/CFS. In addition to general hypocortisolism, ME/CFS patients have a decreased cortisol awakening response, ie, morning cortisol levels peak later compared to healthy controls.

In 2018, Roerink, et al, studied hair cortisol concentrations in CFS patients to assess if hypocortisolism was a prolonged phemenemon. They found that there was a trend of lower hair cortisol concentrations in CFS patients, thus suggesting that hypocortisolism was long-term in ME/CFS.

Notable studies

 * 2018, Hair and salivary cortisol in a cohort of women with chronic fatigue syndrome (Abstract)
 * 2015, Salivary cortisol responses to household tasks among couples with unexplained chronic fatigue
 * 1998, Urinary free cortisol excretion in chronic fatigue syndrome, major depression and in healthy volunteers

Learn more

 * Wikipedia - Cortisol
 * All You Need to Know About Cortisol and Its Health Effects