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What Causes Cushing’s Syndrome?

The hypothalamus sends CRH to the pituitary, which responds by secreting ACTH.  ACTH then causes the adrenals to release cortisol into the bloodstream.
The hypothalamus sends CRH to the pituitary, which responds by secreting ACTH. ACTH then causes the adrenals to release cortisol into the bloodstream.
Image: National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health

Benign Pituitary Tumor (Adenoma)

This is the most common cause of CS. Almost 75 out of 100 to 85 out of 100 all Cushing's syndrome cases are this type. Corticotropin (CRH) is normally made by the pituitary gland. It is a hormone that stimulates the adrenal gland to release cortisol, especially in times of stress. A pituitary tumor causes too much corticotropin (also called ACTH) to be made. ACTH is a kind of pituitary control switch that manages the growth and activity of the adrenal glands. Too much corticotropin causes the adrenal gland to make too much cortisol. Too much cortisol can cause CS.

Adrenal Tumor

This important cause of CS is when a tumor is found in the adrenal gland. The tumor may or may not be cancerous.

Ectopic (Abnormal) ACTH

This is when CS is caused by tumors that make ACTH in other organs. For example, tumors found in the chest on the lung or thymus can make ACTH.

Steroids

Steroids that are used at high doses and for long periods of time may cause CS. For example, the use of prednisone.

It is normal to find high levels of cortisol in the body from stress. This does not lead to Cushing's syndrome.

In very rare cases, there is a hereditary link to an endocrine tumor that could cause CS. Most cases of CS are not passed down.