First the biochemical diagnosis must be made. The adrenal cortical hormone levels must be shown to be lowered.
Circadian rhythm of cortisol - click to enlarge
In primary adrenal insufficiency, basal early morning plasma ACTH concentrations are elevated compared to the low normal levels found in secondary and tertiary disease.
Primary adrenal insufficiency can also be distinguished from secondary and tertiary by using a prolonged ACTH stimulation test. Stimulation of the adrenal gland in primary disease will not result in the release of cortisol. In secondary and tertiary insufficiency the atrophic adrenal glands still produce some cortisol when exposed to ACTH.
Secondary can be distinguished from tertiary adrenal insufficiency by monitoring the ACTH response to stimulation with CRH. There is little or no ACTH response in patients with secondary adrenal insufficiency, but there is a significant ACTH response in patients with tertiary adrenal insufficiency.