What is the Dexamethasone Suppression Test (DST)?
The dexamethasone suppression test measures the response of the adrenal glands to ACTH. Dexamethasone is given and cortisol levels are measured. Cortisol levels should decrease in response to dexamethasone administration. This test measures cortisol before and after dexamethasone to see if your cortisol levels are under normal physical control. In normal individuals, cortisol levels must be suppressed in a “post” pattern.
There are no dietary requirements for this test. You must make an appointment. To find your closest Collection Center and to make an appointment, please refer to your request form, visit our Collection Center locations or visit Telephone Patient Assistance at 1300 732 030.
You must report to the exam at the Collection Center at 9 am for two consecutive hours. If your doctor does not prescribe a dose, you will be given two tablets (1 mg dexamethasone) on the first day at 11 pm. The tablets should be stored in a cool, dark place until taken. If you take the tablets at 11 a.m. On the second day, the collector will certify with you if necessary.
Why do the dexamethasone suppression test?
This test is done when excessive cortisol production is suspected. Low-dose tests can help differentiate healthy people from those who produce more cortisol. An overdose test can help detect abnormalities in the pituitary gland (Cushing’s disease).
ACTH secretion from the pituitary gland is usually regulated by the level of cortisol in the blood plasma. ACTH stimulates the adrenal cortex to produce cortisol. As plasma cortisol levels rise, ACTH secretion is suppressed. ACTH increases as cortisol levels decrease.
Dexamethasone is a synthetic cortisol-like steroid that suppresses ACTH secretion in the general population. Therefore, giving dexamethasone lowers ACTH levels, resulting in lower cortisol levels. People with pituitary glands that produce high levels of ACTH have an abnormal response to a low dose test but have a normal response to a high dose.
- Overnight: 8 a.m. Plasma cortisol <5 mcg / dl
- Dose: Day 3 <10 mcg / day Urine free cortisol
- Overnight:> 50% reduction in plasma cortisol
- Criterion:> 90% reduction in urinary free cortisol
Normal value ranges may vary slightly in different laboratories.
When is a high-dose dexamethasone suppression test needed?
With the occasional pituitary form of Cushing’s disease, cortisol does not suppress blood levels (because the negative feedback loop is more resistant to the effects of synthetic cortisol). Therefore, a dexamethasone overdose (high dose dexamethasone suppression test) is required to overcome this resistance and demonstrate a negative feedback approach.
Cushing’s disease caused by an adrenal gland tumor is resistant to the effects of low and high doses of dexamethasone. Therefore, the decrease in blood cortisol levels after the use of low and high doses of dexamethasone suppression test may support the adrenal origin of Cushing’s disease.
In summary, if cortisol levels are reduced 4 and / or 8 hours after the dose of dexamethasone, we have identified Cushing’s disease of pituitary origin. If the levels do not deviate from the high values of the pre-injection sample, then the diagnosis is of adrenal origin.
Preparation for the dexamethasone suppression test
Before the dexamethasone suppression test, your doctor will instruct you to stop taking certain prescriptions that can affect the results. In addition to:
- Birth control pills
- Phenytoin, which is used to treat seizures.
- Spironolactone, which is used to treat circulatory cirrhosis, ascites, or kidney problems.
- Tetracycline, which is an antibiotic.
How is the dexamethasone suppression test performed?
Two variants of the dexamethasone suppression test are the low-dose test and the high-dose test. Both forms of testing can be done overnight or over a three-day period. The standard test for both is a three-day trial. During both forms of testing, your healthcare provider will administer a certain amount of dexamethasone and then measure your cortisol levels. A blood sample is also required.
Blood is drawn from a vein on the inside of the lower arm or on the back of the hand. First, your healthcare provider will clean the site with disinfectant. They wrap an elastic band around your upper arm to make the vein fill with blood, making it more visible. Next, your healthcare provider will insert a fine needle into the vein and collect a blood sample in a tube attached to the needle. The band is removed and gauze is applied to the site to prevent further bleeding.
Low-dose night test
- Your doctor will give you 1 milligram of dexamethasone at 11 p.m.
- A blood sample will be taken at 8 am the next morning to test your cortisol levels.
- Low-dose standard test
- Collects urine samples in three days and stores them in collection bottles 24 hours a day.
- On the second day, your doctor will give you 48 mg of oral dexamethasone every six hours for 48 hours.
Night overdose test
- Your healthcare provider’s test measures your cortisol levels in the morning.
- You will be given 8 milligrams of dexamethasone at 11 PM.
- Your healthcare provider will take a blood sample at 8 a.m. to measure your cortisol levels.
Standard overdose test
- Collects urine samples in three days and stores them in containers for 24 hours.
- On the second day, your healthcare provider will give you 2 milligrams of oral dexamethasone every 6 hours for 48 hours.
Understand the results of the dexamethasone suppression test
An abnormal low-dose test result indicates that you are experiencing excessive cortisol release. This is called Cushing’s syndrome. This disorder produces ACTH due to an adrenal tumor, a pituitary tumor, or a tumor in another part of the body. The results of overdose tests can help isolate the cause of Cushing’s syndrome.
High cortisol levels can also be caused by other conditions, including:
- Heart attack
- Heart failure
- Poor diet
- The most active thyroid gland
- Anorexia nervosa
- Untreated diabetes
What are the risk factors of the dexamethasone suppression test?
As with any blood draw, there is a risk of minor injury at the injection site. In rare cases, the vein may swell after blood is drawn. This condition, called phlebitis, can be treated with warm compresses several times a day. Ongoing bleeding can be a problem if you have a bleeding defect or are taking blood thinners such as warfarin (a staple product) or aspirin.