The most common cause for Cushing's disease is a pituitary tumor. The pituitary tumor stimulates the adrenal glands to produce too much cortisol.

The important functions of cortisol include:

• Assisting the body in its response to stress
• Carbohydrate and fat metabolism
• Helping balance the effects of insulin in breaking down sugar for energy
• Helping the immune system's response
• Maintaining blood pressure
• Regulation of protein

Additional causes of Cushing's disease include:

• Adrenal tumor
• Small cell carcinoma of the lung
• Bronchial carcinoid tumor