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Adrenocorticotropic hormone, or ACTH, is produced by the pituitary gland in the brain and travels in the blood to the adrenal glands, which sit on top of the kidneys. The hormone acts on the outer part of each adrenal gland, known as the adrenal cortex, causing it to secrete adrenal hormones called glucocorticoids, mineralocorticoids and sex hormones. Glucocorticoids, such as cortisol, affect how the body metabolizes protein and carbohydrates, while mineralocorticoids affect the body's balance of fluid and electrolytes. ACTH secretion increases under conditions of stress.
When there are fewer glucocorticoids secreted by the adrenal cortex, this causes part of the brain known as the hypothalamus to produce corticotropin-releasing hormone. Corticotropin-releasing hormone then travels to the pituitary and stimulates it to make more adrenocorticotropic hormones, increasing adrenal secretion. Higher blood levels of glucocorticoids inhibit the production of corticotropin-releasing hormone, which means less stimulation of the pituitary and a subsequent drop in adrenocorticotropic hormone. This type of system is known as a negative feedback loop.
An adrenocorticotropic hormone test may be carried out to assess the function of the adrenal glands or the pituitary gland. Usually, cortisol levels are also measured to establish a full picture of what is going on. Adrenocorticotropic hormone deficiency can be the result of a tumor of the adrenal gland or an underactive pituitary, and also occurs in some people taking steroid medication.
Where there is an excess of adrenocorticotropic hormone, this can indicate the presence of a hormone-producing cancer in the lungs. Alternatively, it could mean a person has Addison's disease, where the adrenal glands are damaged, or Cushing's disease, in which a benign tumor forms in the pituitary gland and produces adrenocorticotropic hormone. Addison's disease is normally treated by lifelong replacement of the missing adrenal hormones, while in Cushing's disease surgery is used to remove the pituitary tumor.
What is known as an adrenocorticotropic hormone stimulation test may be used to diagnose Addison's disease. In the short version of the test, an injection is given of a substance known as tetracosacside, which is a synthetic copy of adrenocorticotropic hormone. Blood levels of cortisol are measured after about a half-hour has elapsed and compared with levels taken before the test. In a longer version of the test, cortisol levels are measured regularly over the course of 24 hours. If the adrenal glands are functioning normally, they should respond to the tetracosacside by producing more cortisol, so if levels fail to rise this can indicate the adrenal damage seen in Addison's disease.