I am on hydrocortisone, too. But mainly to keep my mastcells in check, they behave especially bad when my cortisol levels are on the lower end. I am taking between 5 and 10 mg, but 10 mg is already interfering with the feedback mechanism in the brain, so I am not taking this on a daily base.
Also my blood cortisol is normal, only free salivary cortisol is a bit low (but still in the normal range).
I think 5 mg wont interfere with the natural feedback mechanism, but everything above 10 mg probably will, especially as cfs folks have hypersensitive feedback mechanisms (that means when the hypothalamus senses theres more cortisol around than "normal" it regulates down the hpa axis even more, shutting down hypothalamus & pituitary signaling, so the adrenals wont receive any signal to produce cortisol). That's the reason why long term use of cortisol can knock out the adrenals. The stress axis shuts down, no signal for the adrenals and then the cortex of the adrenals will shrink. This threshold when the hpa axis shuts down is pretty individual, but for cfs people it's usually lower!
That's also called the "setpoint". Lets say (fictive number) normal cortisol levels are 2-10. WHen a healthy person takes cortisone and then has a number of 11 the hypothalamus will still tolerate this. But let's say at a number of 15 it decices that's enough and to protect the body from too much cortisol he reduces signaling.
In cfs folks this setpoint can be very low. Lets say one has a number of 3 (lower end) and will push this with some hydrocortisone to a 7 thats still inside the reference range and a physiological number. But as the setpoint will maybe only tolerate a 6 that means the hypothalamus will shut down the signaling.
Viruses and cytokines and stress....all that floods the body with tons of cortisol. In the long term the brain reacts to this with downregulation of the stress axis. This is also kind of a genetic and individual make up. So the cfs sufferer is left with lower hypothalamus & pituitary signaling and, as a result, low cortisol (some cfs peops also have high cortisol, but most often its low).
Thats why substituting with hydrocortisone is a tricky thing!