I agree with May, my son does not make any cortisol and has had many tests. You can most definitely trace cortisol in the blood when taking hydrocortisone, my endocrinologist has confirmed this.  I cannot understand why this person should say this, I have data to prove it. Perhaps what she means is that a normal person would not have cortisol around at 3 am in the morning? I would ask for a second opinion or for her to clarify this again.  It would depend on what time the tablet was taken as to how much would be left in the blood system. The centre where we attend do 24 hour profiles which trace the cortisol and 17 OHP for 24 hours.  If you take a tablet at say midnight, it would depend on how much is left in the system at 3 am, as to how much cortisol there is measurable, it would be as accurate as any other time of the day. I would be very interested to hear this Carol VanRyzin comments?  Is she an ped endo?