What is the characteristic symptom of primary adrenal insufficiency?

In primary adrenal insufficiency, also known as Addison's disease, one of the hallmark symptoms is hyperpigmentation of the skin and mucous membranes. This occurs due to an increase in the production of adrenocorticotropic hormone (ACTH) as the body attempts to stimulate the adrenal glands to produce more cortisol. ACTH shares structural similarities with melanocyte-stimulating hormone (MSH), which promotes melanin production in the skin, leading to the characteristic darkening of the skin, especially in areas exposed to friction, such as elbows, knees, and the backs of the hands, as well as mucous membranes.

Understanding this mechanism can help differentiate primary adrenal insufficiency from other conditions. For instance, weight gain is more commonly associated with conditions like hypercortisolism (Cushing's syndrome) rather than adrenal insufficiency. Edema can occur in various conditions, but it is not a key feature of primary adrenal insufficiency. Acne is also unrelated and is not a symptom linked specifically to this condition. Thus, hyperpigmentation stands out as the defining symptom in the context of primary adrenal insufficiency.