Presentation with hyperpigmentation and hypotension are features strongly suggesting primary adrenal insufficiency.
Hypotension is likely the result of failure of the renin-angiotensin-aldosterone system (RAAS). The adrenal glands are unable to upregulate aldosterone secretion. This results in increased sodium loss and potassium reabsorption as well as intravascular volume depletion, decreased vascular tone and hypotension.
In secondary adrenal insufficiency, the RAAS is intact and aldosterone is able to be upregulated and resistant hypotension is far less common.
Hyperpigmentation in primary insufficiency is the result of increased production of ACTH. ACTH is generated by cleaving the prohormone proopiomelanocortin into ACTH and melanocyte stimulating hormone (MSH).
High levels of MSH cause melanin deposition. The most common sites to see this are exposed areas and those which undergo trauma (for example, knuckles, buccal mucosa, recent scars, palmar creases, pressure areas etc).
In secondary and tertiary adrenal insufficiency ACTH levels are reduced and as such to not result in elevated MSH levels.
