Research into the stress response was pioneered by physiologist Hans Seyle who called it:

The General Adaptation Syndrome (GAS)

There are three stages to the GAS or stress response.

1) The Alarm stage

When our body perceives a stress, signals are sent down the Hypothalamus Pituitary Adrenal Axis (HPA axis) to the end organ, the adrenal glands.
These glands produce the stress hormones: adrenaline the effects of which may only last minutes and cortisol whose effects may last 12-15 hours after the onset of the stress. This is the ‘fight or flight’ response.

When the alarm reaction is over there is a recovery stage of maybe a few days while the body regains homeostasis.

If after this recovery phase there is an additional stress or series of stresses we go into the second stage of GAS this is:

2) The Adaptive or Resistance Stage

In this stage levels of cortisol are raised above normal. When this goes on for an extended period we experience the negative effects of cortisol, among these are insomnia, anxiety, panic attacks, inhibition of the conversion of thyroxine (T4) to the active thyroid hormone (T3) and our immune system becomes depressed. This is chronic stress.
This stage can last for months or even up to 20 years, it rather depends on how good your adrenal glands are. ME/CFS people who have good periods and then relapses are going in & out of this stage. If it is recognized and appropriate steps are taken you can recover. People who are helped by Reverse, Mickel or Lightning Process are may be at this stage. Those who are more severely affected have progressed to the third stage of the GAS which is:

3) The Exhaustion Stage (often termed ‘burnout’)

Here your body can no longer respond to the high metabolic demands of chronic stress. The adrenals can no longer produce sufficient cortisol even for the normal unstressed day, and you cannot get out of bed. This will ultimately cause thyroid dysfunction, because adequate cortisol levels are needed to admit thyroid hormone into the cells. This exhaustion stage is where Addison’s disease sufferers are. They however are here because something has destroyed the outer part(cortex) of their adrenal glands which produces cortisol. This tissue cannot recover and their only option is to take hydrocortisone permanently in the doses their adrenals should produce and extra when a stress like a cold or flu occur. This stage is also where some severely affected ME/CFS person may be. However their adrenal cortex is NOT destroyed but exhausted by chronic stress. They too will need hydrocortisone to alleviate the pressure on their adrenals but this may be for a temporary period of a few years.

SO, CHRONIC ILLNESS OF ANY KIND EQUALS CHRONIC STRESS.

Sadly GP’s and endrocrinologists on the whole are unaware of how ill you can be in the second stage of GAS. The NHS does not use the 24 hour saliva tests which are available privately but a random cortisol, which gives no real information as the adrenals have a 24 hour rhythm.