Adrenal insufficiency is a disease of the endocrine system in which the adrenal glands, located on top of each kidney, do not make enough hormones. There are two types of adrenal insufficiency: primary and secondary. Primary adrenal insufficiency is referred to as Addison's disease, where the glands are damaged and do not synthesize enough cortisol and aldosterone. These hormones are important for regulating blood pressure and metabolism and are also key responders to stress. Secondary adrenal insufficiency is the most common kind, and it begins in the pea-sized pituitary gland located in the brainstem. With this type, the pituitary gland does not make adrenocorticotropin (ACTH), which is necessary for the adrenal glands to produce cortisol.
Hyperpigmentation occurs only with Addison's disease, which, as mentioned, is the primary form of adrenal insufficiency. Since Addison's disease involves damaged adrenal glands that do not make enough hormones, the pituitary gland responds by making more of the adrenal-stimulating hormone ACTH. ACTH is directly involved in making melanin, the hormone responsible for darkening the skin. As melanin increases in the body, it causes darkening of the skin. It is often seen around joints, scar tissue, and mucous membranes. It can also cause the lips, rectum, and lining of the mouth to turn a bluish-black. Black freckles are usually found on the face, neck, and shoulders. Areas of the body not normally in the sunlight will still darken, but parts of the body in the sunlight will become even darker.