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What is the difference between high dose and low dose dexamethason suppression test in Cushing Syndrome diagnosis?

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In a dexamethasone supression test, it is measured whether given the patient dexamethasone leads to lowered cortisol levels. Lowered cortisol levels are the normal response to dexamethasone; if the level doesn't go down as much as it should, that can point to one of several conditions that cause Cushing's syndrome (see below).

There are standard and overnight dexamethason tests, and high dose and low dose tests. In the standard test, measurements are done over three days - in the low dose test, 0.5 mg of dexamethasone are given every six hours from day 2 onward; in the high dose test, it's 2mg. In the overnight test, in the low dose variety, the patient is given 1mg of dexamethason, and in a high dose dexamethason test, the patient is given 8mg.

Both tests can determine whether the patient's cortisol release is abnormal. This is usually due to one of three conditions:

  1. Cushing's Disease (a pituitary tumor)
  2. an adrenal tumor
  3. or a tumor somewhere else in the body that produces a hormone called adrenocorticotropic hormone.

With the high dose test, it is possible to identify whether the problem is Cushing's disease, while the low dose test can't make that distinction.

Sources:

US National Library of Medicine - Dexamethasone suppression test

UCLA Endocrine Surgery Patient Education

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