Cushing's syndrome (cortisol-producing adrenal tumor)
What is Cushing's syndrome?
Figure 4: Harvey Cushing
Cushing's syndromeCushing's syndrome - a disease where too much cortisol is being made is named after Harvey Cushing, a famous neurosurgeon at Johns Hopkins Hospital who first described it. Patients who have Cushing's syndrome have too much cortisolCortisol - a glucocorticoid that controls glucose production and suppresses the immune system or steroid in their blood. Cortisol is a hormoneHormone - a chemical made by a gland that travels in the bloodstream and "tells organs what to do." made by the adrenal glands that plays an essential role in the stress response. Normal cortisol levels are necessary to sustain life, to maintain normal sleep-wake cycles, and to enable the body to respond to stressful events. Though limited bursts of cortisol are normal, long-term elevations of the cortisol level are harmful to many organ systems. Cushing's syndrome can be caused by either taking too much steroids as a medication or if the body makes too much steroid.
People who take prescription medications similar to cortisol in high doses for a long period of time for inflammatory diseases (such as rheumatoid arthritis, asthma, and inflammatory bowel disease) may develop Cushing's syndrome. In fact, this is the most common cause of Cushing's syndrome and is termed "iatrogenic", i.e. caused by the doctor. Medications that are similar to cortisol are called glucocorticoids, corticosteroids, or sometimes just "steroids" for short. They are not the same as the performance-enhancing steroids taken by some athletes. The most commonly prescribed glucocorticoid medication is called prednisone. Because glucocorticoids suppress the immune system, they reduce inflammation and are also frequently given to people who have had organ transplants to prevent the immune cells from attacking the transplant.
If the patient's body is making too much cortisol (i.e. endogenous Cushing's syndrome), there are 3 possible sources:
- Pituitary source A small tumor in the pituitary gland (the "master gland" that lies at the base of the brain), is secreting too much ACTH (adrenocorticotropic hormone), a hormone that controls the activity of the adrenal glands. The ACTH signal travels through the bloodstream and causes the normal adrenal glands to make too much cortisol. This can lead to hyperactivity of both adrenal glands called bilateral adrenal hyperplasia. Pituitary tumors are the most common cause of endogenous Cushing's syndrome, making up about 70% of cases. When the pituitary is the source of ACTH, it is actually called Cushing's disease rather than Cushing's syndrome.
- Adrenal source A tumor in one of the adrenal glands is producing too much cortisol. In this scenario, the adrenal tumor acts alone and the pituitary gland is normal. Adrenal tumors are responsible for endogenous Cushing's syndrome in about 30% of cases.
- Other "ectopic" source This very rare condition, called ectopic ("outside location") ACTH syndrome, accounts for less than 1% of cases. It means that a tumor lying outside of the pituitary gland is making too much ACTH. The most common places for an ectopic ACTH-producing tumor are in the lung and thymus, both located in the chest. Other very rare places include the thyroid, ovary, adrenal gland, and liver. Just like a pituitary adenoma, these ACTH-producing tumors make too much ACTH and in turn cause the adrenal glands to make too much cortisol.
The causes of Cushing's syndrome can be divided into:
- ACTH-dependent disease In these diseases, the body is making too much ACTH which leads to too much cortisol production. Pituitary tumors and ectopic ACTH producing tumors are examples.
- ACTH-independent disease In this case, the adrenal gland is making too much cortisol and the ACTH is low. This is usually caused by an adrenal tumor but can be caused by bilateral adrenal hyperplasia as well.