Cortisol over 800 excludes a problem, normal response to Syncathen test (ACTH) at 1 hr should be over 470nmol/l (over 6yrs), 650 (under 6yrs).
Primary adrenal insufficiency – congenital adrenal hyperplasia (mostly 21OH deficiency), else autoimmune, genetic, infiltrative, adrenleukodystrophy (and exogenous steroid suppression).
Classic Addisonion picture – low sodium, high potassium, hyperpigmentation (can look slim and tanned, so misleadingly healthy!).
Physiological hydrocortisone replacement =10mg/m2/d in 3-4 divided doses. Medication alert bracelet recommended, in case of crisis. Monitor growth, BP.
If unwell, double HC dose (if varying doses, double highest dose) and give it three times daily. IM if necessary, +/- infusion.