Cortisol is a glucocorticoid hormone produced by the cortex of the adrenal glands. It has diverse functions influencing nutrient metabolisms, inflammatory processes in the body and immunity. Cortisol secretion is regulated by the presence of adrenocorticotropin hormone (ACTH) which is secreted by the anterior pituitary (pituitary gland). This in turn is regulated by corticotropin-releasing factor (CRF) secreted by the hypothalamus. When the cortisol levels in the blood are low, the hypothalamus is stimulated to secrete CRF which then acts on the anterior pituitary to release ACTH. The ACTH then reaches the adrenal gland via the bloodstream and stimulates it to secrete cortisol. If the cortisol levels are high, CRF secretion decreases and so does ACTH.
High cortisol levels are seen in Cushing syndrome while low cortisol levels are seen with adrenal insufficiency. There are various similar disorders of the adrenal gland, pituitary gland or hypothalamus that may also be responsible for high or low cortisol levels. The level of circulating cortisol in the blood fluctuates during the course of a 24 hour period. It is highest around 8 am and lowest around midnight.
Types of Cortisol Tests for Screening and Diagnosis
Cortisol tests are used to measure the level of cortisol in the blood stream. There are three main methods of testing the cortisol levels and this includes a blood test, urine test and saliva test.
Cortisol Blood Test
Blood is usually drawn from the arm (vein) around 8am and again at 4pm. Another sample may again be collected late in the evening. A single morning sample may be sufficient to assess whether the peak cortisol level is close to normal peak levels. However several samples at different time periods will allow the expected variations in cortisol levels to be monitored.
Cortisol Urine Test
A cortisol urine test requires a 24 hour urine sample. This means that urine is collected throughout a 24 hour period in a jar and sent for testing to assess the levels of free cortisol. A 24 hour urine sample does not allow for the assessment of normal cortisol variations during the course of the day. Sometimes a single urine sample taken in the morning may be used to detect lower than normal levels of cortisol when it is expected to be at its peak.
Cortisol Saliva Test
The saliva test is not frequently used. Patients gather a sample of saliva between 11pm and midnight when the cortisol levels are expected to be at it lowest within the 24 hour period. The sample is then sent for testing and ideally several samples should be used. A late-night salivary cortisol measurement is convenient for screening and diagnosing Cushing syndrome. However other tests should also be done to confirm the diagnosis.
Dexamethasone Suppression Test
This test involves administering a synthetic glucocorticoid known as dexamethasone to a patient. The dexamethasone in the blood should normally cause the ACTH levels to drop and therefore the cortisol levels to drop. This will help to ascertain whether high cortisol levels are related to ACTH. There are three types of dexamethasone suppression tests :
- Overnight low-dose dexamethasone suppression : 1mg dexamethasone (orally) at midnight with blood test at 8 to 9am
- 48 hour low-dose dexamethasone suppression : 0.5mg every 6 hours for 48 hours with 24 hour urine sample on second day and blood test at 8 to 9 am after second day.
- 48 hour high dose dexamethasone suppression : 2 mg every 6 hours for 48 hours with 24 hour urine sample prior to test and on second day of test.
Corticotropin-Releasing Hormone (CRH) Test
This test involves administering corticotropin releasing hormone (CRH) intravenously and the monitoring the blood ACTH and cortisol levels for two hours. CRH should increase ACTH levels which should then increase cortisol levels.
A variation of this test known as the dexamethasone-CRH test is done to distinguish between Cushing syndrome and pseudo-Cushing syndrome. This involves the same drug administration as the 48 hour low-dose dexamethasone with CRH then administered intravenously after the last dose of oral dexamethasone (after the 8th dose). ACTH and cortisol levels in the blood are monitored every 15 minutes for 1 hour.
ACTH Stimulation Test
The blood cortisol and aldosterone levels are first taken. Synthetic ACTH (0.25mg) is administered by intramuscular (IM) or intravenous (UV) injection. The cortisol and aldosterone levels are again measured 30 minutes later or sometimes 60 minutes later.
High Cortisol Tests
If Cushing’s syndrome is suspected then the following tests may first be done to confirm the diagnosis.
First an overnight low-dose dexamethasone suppression test is done. If the test does not exclude Cushing syndrome then a 48 hour low-dose dexamethasone suppression test is done.
If both tests then confirm Cushing syndrome it is important to then ascertain whether it is ACTH-dependent or ACTH-independent. At this point the dexamethasone-CRH test may be done to exclude pseudo-Cushing syndrome especially in alcoholics or depressed patients.
Plasma ACTH (ACTH in the blood) is then tested and if it is detectable, possibly indicating ACTH-dependent Cushing syndrome, then a CRH test and 48 hour high dose dexamethasone suppression test should be done. If the plasma ACTH is undetectable indicating possible ACTH-independent Cushing syndrome then adrenal glands need to be scanned for possible adenomas (the most common cause of ACTH-independent Cushing syndrome).
Low Cortisol Tests
If Addison’s disease is suspected then the ACTH stimulation test may be done to confirm the diagnosis. The administration of ACTH should increase the level of cortisol by 2 to 5 times the baseline levels within 30 minutes. The aldosterone levels also rise within this time although ACTH is not the main stimulant for aldosterone secretion.
Dangerously low levels of cortisol may be seen in an adrenal crisis. The patient often has a history of adrenal insufficiency and the blood cortisol levels alone are sufficient to prompt therapy with hydrocortisone or prednisone (corticosteroids). If there is uncertainty, then dexamethasone can be administrated and the cortisol levels tested again after 24 to 36 hours.