From the Mayo Clinic:

• • Cushing syndrome

  Cushing syndrome occurs when your body is exposed to high levels of the hormone cortisol for a long time. Cushing syndrome, sometimes called hypercortisolism, may be caused by the
  

• use of oral corticosteroid medication. The condition can also occur when your body makes too much cortisol on its own.

• Endocrine system

Excess levels of the hormone cortisol are responsible for Cushing syndrome. Cortisol, which is produced in the adrenal glands, plays a variety of roles in your body. For example, cortisol helps regulate your blood pressure and keeps your cardiovascular system functioning normally.

Cortisol also helps your body respond to stress and regulates the way you convert (metabolize) proteins, carbohydrates and fats in your diet into usable energy. However, when the level of cortisol is too high in your body, you may develop Cushing syndrome.

Your body's own overproduction

The condition may also be due to your body's own overproduction of cortisol (endogenous Cushing syndrome). This may occur from excess production by one or both adrenal glands, or overproduction of the adrenocorticotropic hormone (ACTH), which normally regulates cortisol production, In these cases, Cushing syndrome may be related to:

• A pituitary gland tumor (pituitary adenoma). A noncancerous (benign) tumor of the pituitary gland, located at the base of the brain, secretes an excess amount of ACTH, which in turn stimulates the adrenal glands to make more cortisol. When this form of the syndrome develops, it's called Cushing disease. It occurs much more often in women and is the most common form of endogenous Cushing syndrome.

• An ectopic ACTH-secreting tumor. Rarely, when a tumor develops in an organ that normally does not produce ACTH, the tumor will begin to secrete this hormone in excess, resulting in Cushing syndrome. These tumors, which can be noncancerous (benign) or cancerous (malignant), are usually found in the lungs, pancreas, thyroid or thymus gland.

• A primary adrenal gland disease. In some people, the cause of Cushing syndrome is excess cortisol secretion that doesn't depend on stimulation from ACTH and is associated with disorders of the adrenal glands. The most common of these disorders is a noncancerous tumor of the adrenal cortex, called an adrenal adenoma.