My cortisol level is 6.7. it's way low in normal range. reason for concern?

Probably not...: ... But the more important question is why was the cortisol level drawn in the first place? Reference ranges for cortisol are almost meaningless as cortisol levels are affected by so many things... Including time of day, one's stress level, etc. Having said that, in some contexts, an 8am cortisol value of 6.7 mcg/dl might be considered a bit low. Best to get the advice of the doctor who drew it.