What is a dexamethasone suppression test used for?

A dexamethasone suppression test is a blood test used to determine if you have Cushing syndrome. Cushing syndrome occurs when too much cortisol is made by your adrenal glands. Cortisol is a hormone that helps you respond to stress, but too much cortisol can lead to health problems.

In people with Cushing syndrome, the levels of cortisol levels are abnormally high after the dexamethasone suppression test. You may have too much cortisol due to a medical problem or because you have been taking medicines called glucocorticoids (like prednisone) which affect the body similar to cortisol.

Without treatment, Cushing syndrome can cause complications like bone loss, high blood pressure, type 2 diabetes, infections, or muscle weakness.

Your healthcare provider may use blood, urine, or saliva tests to help diagnose Cushing’s syndrome.

What is cortisol?

Cortisol is an important hormone produced by the body that helps to control blood pressure, maintain heart and blood vessel function, lower inflammation and regulate blood glucose (sugar). It also helps the body to utilize food to provide energy.

Cortisol is made in the adrenal glands found just above your kidneys. Its production is controlled by another hormone called adrenocorticotropic hormone (ACTH) made by your pituitary gland.

What are alternative names for the dexamethasone suppression test?

Other names used for the dexamethasone suppression test may include:

• ACTH suppression test
• DST
• Cortisol suppression test
• Overnight dexamethasone suppression test
• Low-dose dexamethasone suppression test (LDDST)

What can cause Cushing syndrome?

• Pituitary tumor: A tumor in your pituitary gland can also cause high levels of cortisol and is called Cushing disease. Your pituitary gland, found at the base of your brain, makes adrenocorticotropic hormone (ACTH for short). ACTH helps your adrenal glands to make cortisol. A tumor in this area will increase cortisol levels. These tumors are not cancerous.
• Adrenal gland tumor: A tumor on the outer part of your adrenal glands may cause high levels of cortisol. This is called Cushing syndrome. These tumors are not cancerous.
• Ectopic tumors: Some organs can have tumors that make high levels of ACTH and increase cortisol levels. These organs do not normally make ACTH. The tumors may or may not be cancerous. It can occur in organs like the lungs, thyroid gland or pancreas.
• Use of glucocorticoids: Long-term use of corticosteroid medicines (like prednisone, methylprednisolone, dexamethasone or hydrocortisone) may cause high cortisol levels and lead to Cushing syndrome.
• Inherited: Rarely, Cushing syndrome can be inherited through the genes.

What are the symptoms of Cushing syndrome?

The most common symptoms of Cushing syndrome include:

• Increased fat around the neck or shoulder area, which may cause a fatty hump between the shoulders (“buffalo hump”), a round or reddish face (“moon face”), or weight gain in the stomach area. Your limbs may become thinner.
• Thin and fragile skin with red, pink or purple stretch marks (striae) on the skin (stomach, hips, buttocks, thighs, breasts or underarms).

You may also have:

• Acne
• Slow wound healing
• Elevated blood glucose (blood sugar), which may lead to diabetes
• Excessive body hair (hirsutism) in females
• Irregularity periods in females
• High blood pressure
• Insomnia (trouble sleeping)
• Bone loss

Other symptoms can include: blood clots, infection, fatigue, skin darkening, slow wound healing, mood swings, lowered sex drive, erectile dysfunction (in men), depression, panic attacks or anxiety, muscle weakness, slowed growth in children.

People who have been diagnosed with Cushing syndrome can usually be treated with effective therapy, surgery or cured. Treatments lower the cortisol levels and improve the symptoms.

Why is dexamethasone used for this test?

Dexamethasone is a man-made corticosteroid (“steroid”) pill that binds to the same area in the body as cortisol. Normally, your cortisol levels should decrease after receiving the dexamethasone pill. In people with Cushing syndrome, measured cortisol levels can be high after taking dexamethasone.

Dexamethasone is about 30 to 40 times more potent than cortisol. It binds strongly to the glucocorticoid receptors in the body and has a long duration of action, with a biological half-life of about 36 to 54 hours. In addition, dexamethasone does not interfere with cortisol measurements in the plasma, urine or saliva. These qualities make dexamethasone an ideal steroid to assess cortisol levels.

Other uses of dexamethasone

When a corticosteroid is used as a medicine to treat disease, it may be used to help lower inflammation after an injury or for conditions like arthritis, severe allergies, asthma, inflammatory bowel disease, lupus or cancer. They may also be used to help prevent organ rejection after a transplant.

Besides dexamethasone, other common corticosteroids include prednisone, methylprednisolone, or hydrocortisone.

How is the dexamethasone suppression test performed?

Most commonly, the dexamethasone suppression test is given using a low 1 mg dose of dexamethasone given at night, then your blood is drawn early the next morning. Doses are modified in children.

• For the low-dose, overnight dexamethasone suppression test, you will take an oral 1 mg dexamethasone pill between 11 PM and midnight, as directed by your doctor. Dexamethasone is a corticosteroid (“steroid”) pill.
• Between 8 and 9 AM the next morning, you will have your blood drawn at a lab. If your cortisol levels are high, it may mean you have Cushing syndrome, which can be due to a variety of reasons.
• Your test results should be available in a few days. Your doctor will contact you to discuss your results and next steps.

How is the blood test done?

The blood test is done using a needle to draw blood, usually from a vein in your arm. Most people only feel a small amount of pain for a short while.

You may have a small amount of bleeding or bruising afterwards, but the risk is usually small. The laboratory technician can give you a pressure bandage to help quickly stop any bleeding.

Other, less common risks from having your blood drawn include:

• Abnormal bleeding
• Fainting or dizziness
• Phlebitis (inflammation inside the vein)
• Hematoma (collection of blood under the skin)
• Infection

If you experience any symptoms that concern you, contact your healthcare provider.

Your healthcare provider may tell you need to stop taking some types of medicines that may interfere with the test, including:

• Antibiotics
• Birth control pills or estrogen
• Other corticosteroid medicines like prednisone, methylprednisolone or hydrocortisone
• Seizure medicines
• Water pills (diuretics)
• Certain pain medicines

Do not stop any medicines unless directed to do so by your doctor.

Types of dexamethasone suppression tests

In people with Cushing syndrome, cortisol levels are abnormally high after the dexamethasone suppression test (DST).

Low dose (1 mg), overnight dexamethasone suppression test

• For the low-dose, overnight test, adults are given 1 milligram (mg) of oral dexamethasone at 11 PM.
• At 8 AM the next day a healthcare provider will check your cortisol level by drawing a blood sample from a vein in your arm. This blood test will measure cortisol levels.
• Your doctor may order dexamethasone and ACTH levels, too.

ACTH is a hormone your pituitary gland releases that tells your adrenal glands to release cortisol. ACTH is made by the small pituitary gland found at the base of your brain. Normally, when you take dexamethasone, ACTH production is reduced, which then leads to lower cortisol levels.

If your pituitary gland makes too much ACTH, you will have an abnormal response to the low-dose overnight dexamethasone suppression test, and the blood test will show that your cortisol levels are high.

Two-day, low-dose dexamethasone suppression test

A two-day, low-dose test may be used as well to check for high cortisol levels. This may be used if results of the overnight test are not clear, or as an alternative test.

• For this test, dexamethasone 0.5 mg is taken every 6 hours by mouth for 2 days (8 AM, 2 PM, 8 PM and 2 AM) for a total of eight doses (2 mg each day).
• Blood is drawn 2 or 6 hours after the last dose to measure cortisol.
• Your doctor may order dexamethasone and ACTH levels, too.

High dose (8 mg), overnight dexamethasone suppression test

The high dose test is done to determine if high cortisol levels are occurring from a problem with the pituitary gland (Cushing disease) or from an ectopic site in your body (meaning somewhere else). This test is not for routine use and should be interpreted with caution also using an MRI and clinical features.

• For the high-dose, 8 mg overnight test, you will first have your blood drawn on the morning of the test (8:30 AM) to measure your cortisol levels for comparison. Then at 11 PM that night you will take 8 mg of oral dexamethasone.
• At 8:30 AM the next morning, your blood is drawn again to check your cortisol levels.
• Your doctor may order dexamethasone and ACTH levels, too.

The high-dose dexamethasone suppression test may also be done over 2 days, utilizing a total of 16 mg over 2 days.

Standard urine tests

Less commonly, a 3-day test may be performed to analyze cortisol levels in your urine. Your urine will be analyzed instead of using the blood test. For a urine test, you may need to save your urine and send the sample to a lab. These types of tests are not commonly used.

Other tests that may be used to diagnose Cushing syndrome include a saliva test, CT scan, MRI or special tests to sample blood that drains from the pituitary gland (inferior petrosal sinus sampling).

High-dose dexamethasone suppression test (HDDST). This test is the same as the LDDST, except it uses higher doses of dexamethasone. If the cortisol levels in your blood drop after taking a high dose of dexamethasone, you probably have a pituitary tumor. If your cortisol levels don’t drop, you may have an ectopic tumor.

What are normal results?

Normal results typically mean that you do not have Cushing syndrome.

Normally, your cortisol levels should decrease after receiving the dexamethasone pill. This is because dexamethasone and cortisol are similar chemicals and dexamethasone will suppress your body’s natural production of cortisol.

Normal results for the low dose, 1 mg overnight DST and the two-day, 2 mg DST:

• The 8 AM blood test shows that your cortisol levels are less than 1.8 micrograms per deciliter (mcg/dL) or less than 50 nanomoles per liter (nmol/L).

Normal results for the high-dose (8 mg dexamethasone) overnight test:

• The 8 AM blood test shows that there is more than a 50% reduction in your cortisol levels.

Your healthcare provider may also measure your ACTH levels or dexamethasone levels at the same time to help interpret the test and determine the cause of high cortisol.

False-positive dexamethasone suppression test results could occur due to low dexamethasone levels (either because the medicine was not taken or its metabolism was altered, possibly by another drug).

Test values and normal results may differ among laboratories, so be sure to talk to your doctor to interpret your specific numbers.

What do abnormal results mean?

There are different reasons why your body may be making too much cortisol. If your levels are over 1.8 mcg/dL, your healthcare provider may want to confirm the results with a second test.

If your results are abnormal, your blood tests may show:

Adrenal gland tumor

• The adrenal glands are small glands that are found on the top of your kidneys.
• The low-dose blood test show you have no decrease in blood cortisol and low ACTH levels. This means you may have an adrenal gland tumor that produces cortisol.
• A high dose test is not normally needed.

Pituitary gland tumor (Cushing disease)

• The pituitary gland is a small gland found at the base of your brain
• The low-dose blood test shows you have no decrease in blood cortisol. The high dose test shows a decrease in blood cortisol.
• Levels of ACTH may be normal or high, suggesting a pituitary source.
• You may have a pituitary gland tumor that produces ACTH, which leads to high cortisol levels. This is known as Cushing disease, a type of Cushing syndrome.

Ectopic tumor (Ectopic Cushing syndrome)

• The low-dose blood test show you have no decrease in blood cortisol levels. Your ACTH levels are found to be normal or high.
• Results for the high-dose test show no decrease in blood cortisol.
• You may have an ectopic tumor, which is a tumor found in another part of the body that produces ACTH.

Other conditions that can lead to high cortisol levels include mental health disorders like depression or stress, excessive alcohol use, being overweight or obese, pregnancy, kidney problems, and high blood sugar levels.

Cushing syndrome can be hard to diagnose and your doctor may send you to see an endocrinologist, a type of doctor that specializes in diseases that cause hormone-related imbalances in your body.

How is Cushing syndrome treated?

Treatment for Cushing syndrome depends upon what is causing increased cortisol levels in your body. Treatment is aimed at lowering the cortisol levels.

If you are taking corticosteroid medicines long-term, your dose may need to be slowly changed. Only change your dose as recommended by your doctor.

You may need surgery to remove a tumor that causes high cortisol levels. You may also need to take cortisol replacement therapy after surgery.

Medicine may be needed to block the adrenal glands from making cortisol. Medicine or radiation therapy (high-energy x-rays) may be used to shrink a tumor or kill cancer cells.

Medicines that may be used to control cortisol production or Cushing syndrome symptoms include:

• ketoconazole
• levoketoconazole (Recorlev)
• metyrapone (Metopirone)
• mitotane (Lysodren)
• osilodrostat (Isturisa)
• pasireotide (Signifor)
• mifepristone (Korlym)

This is not all the information you need to know about the dexamethasone suppression test (DST) and Cushing syndrome and does not take the place of your healthcare provider’s directions. Discuss any medical questions you have with your doctor or other health care provider.