If cortisol is shown to be low on testing, I think it is a mistake to only supplement with DHEA and hope that will bring up the cortisol level somehow because it generally won't be able to do so by itself. Most of the time one will need both HC and DHEA to correct a deficiency.
I do agree that there is a stage where cortisol and dhea have a yin and yang relationship but that is not always the case. In later stages of HPA dysfunction, cortisol and DHEA can both be low and there is no evidence that shows that increasing DHEA further lowers cortisol. Instead, the studies that have been done show that those with Addison's disease (ie no cortisol production) do much better when DHEA is added into the mix.
DHEA seems to be a hormone where the form and brand are crucially important. The lipid matrix or micronized forms seem to be vastly superior to other forms in terms of absorption and effect.
What im getting at is i think its a problem increasing cortisol with a low dhea level. need dhea to counteract the catabolism effects of cortisol and other negative effects of cortisol. Cortisol is needed to make energy and help with inflammation.
High cortisol with low dhea just makes one overstimulated and my experience feel like crap, very wired but tired. MAybe one needs to slowly add both, but i found slowly increasing dhea first worked for me. my cortisol hadnt bottomed out as much like my dhea but my cortisol was in the low normal. I initially start with a transdermal cream with both pregnenolone/dhea (preg to help increase cortisol), basically it made me bounce off the walls, virtually no sleep and was very angry like a roid rage. After a break i started supplementing dhea at 5mg with out pregnenolone and slowly increased it in 5mg increments until i got to 25mg a day, this got my dhea levels into the middle of the reference range. Now that im adding pregnenolone to help with cortisol, i am tolerating it alot better. When i had low dhea levels i could barely tolerate 5mg of pregnenolone(preg did increase my cortisol and testosterone, no effect on dhea), my initial bad experience with preg i used 50mg which sent me whacky. Now with good dhea levels i am supplementing pregnenolone 50mg and not having those overstimulated effects but just a good smooth amount of energy. I do think the dhea has protected me from the negative parts of increased cortisol.
I dont think someone with very low cortisol is going have it increase their cortisol levels using dhea, but i think as they increase cortisol levels one may need to start increasing dhea. I am just generalising but i think adrenal hormone treatments need to be tailored to the individual, again we all react very differently.