Normally, the pituitary gland produces ACTH (Adrenocorticotropic hormone), which circulates in the blood and stimulates the adrenal glands to produce cortisol. Right above the pituitary gland, in the base of the brain, is the hypothalamus. The hypothalamus produces CRH (Corticotropin releasing hormone), which stimulates the pituitary gland to produce ACTH. Therefore, hypercortisolism can arise from too much CRH, ACTH; or from excessive production of cortisol by the adrenal gland.
The causes are divided into ACTH dependent and ACTH independent sources.
ACTH dependent: Includes a pituitary tumor producing ACTH (referred to as Cushing's disease and is the most common cause), and ectopic ACTH production (ACTH or CRH is produced elsewhere, such as by a carcinoid tumor or lung cancer).
ACTH independent: Cushing's syndrome consists of an adrenal growth, either benign or malignant, producing cortisol. ACTH levels are low, undetectable if there is an adrenal source.
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