Cushing’s syndrome is not the same as Cushing’s disease. They’re similar but also different conditions: Cushing’s syndrome is less serious and refers to “the general state characterized by excessive levels of cortisol in the blood,” while Cushing’s disease is the condition caused by a pituitary tumor that secretes the hormone ACTH, which causes excess cortisol. (10) Low Cortisol Levels: Addison’s Disease and Adrenal Fatigue On the other hand, the opposite of having Cushing’s disease — experiencing abnormally low cortisol levels — can result in a condition known as Addison’s disease, adrenal insufficiency or adrenal fatigue. Addison’s disease is also rare and is considered a type of autoimmune disease, since it causes the immune system to attack the body’s own healthy tissue. In this case, tissues within the adrenal glands themselves becomes damaged and inflamed, which alters how the adrenals produce .