Testing for Cushing’s syndrome is done in two stages. Initial tests are to verify that there is indeed excess cortisol present. These first stage tests include a 24-hour urine test and a single low dose dexamethasone suppression test. The second set of tests are to determine the cause of the increased cortisol: pituitary, adrenal, or other. These tests include a high dose dexamethasone suppression test, a CRH stimulation test, and one or more imaging tests.

Laboratory Tests
Since cortisol levels vary greatly over the course of a day, a single cortisol result is of little value and other ways of measuring overall cortisol production are used.

24-hour Urine Cortisol
24-hour urine cortisol (or urine free cortisol) is often used to evaluate overall cortisol production.

Another approach is to measure cortisol at 11 PM, when normally levels are at their lowest. This can be done on saliva or in the blood. If these tests indicate that cortisol levels are not varying normally and are consistently high, other testing is done to determine the reason for the increased levels.

Dexamethasone Suppression Test
Dexamethasone is a synthetic steroid that mimics cortisol in the feedback inhibition of CRH and ACTH production. There are several versions of the suppression test, which is used to confirm the diagnosis of Cushing’s syndrome. A normal response to dexamethasone is suppression of cortisol secretion. Patients with all types of Cushing’s syndrome will not show adequate suppression after a single low dose of dexamethasone given at bedtime.

Higher doses of dexamethasone can be given over 48 hours to distinguish between an ACTH-producing pituitary tumor and other causes of Cushing’s syndrome.

CRH Stimulation Test
CRH (corticotrophin releasing hormone) is injected and cortisol and ACTH levels are measured at baseline (before CRH) and at 30 and 60 minutes. The normal response is a peak in ACTH levels at 30 minutes with cortisol peaking at 60 minutes. Most patients with Cushing’s syndrome caused by adrenal tumors or ectopic ACTH secreting tumors do not respond to CRH. Frequently, ACTH levels are measured in samples obtained through a catheter placed in the inferior petrosal sinuses, which carry blood from the pituitary glands.

Non-Laboratory Tests