Dexamethasone Suppression Test

When is the Dexamethasone Suppression Test recommended?

The Dexamethasone Suppression Test is recommended when there is a suspicion of overproduction of cortisol, such as in the case of Cushing's syndrome or Cushing's disease. It is used to evaluate endogenous Cushing syndrome by assessing the lack of suppression of cortisol secretion and to differentiate healthy individuals from those who produce excessive cortisol.

The test is performed by administering dexamethasone, a synthetic glucocorticoid, and measuring the adrenal glands' response to ACTH. There are two types of test: the low-dose test and the high-dose test, which can be performed overnight or over a standard 3-day period. The test helps determine the cause of an excess of cortisol in the body and can aid in diagnosing specific conditions related to cortisol production.

How should patients prepare for the Dexamethasone Suppression Test?

Patients should prepare for the Dexamethasone Suppression Test by following these guidelines:

• Continue taking usual medications: Patients can eat, drink normally, and take all their regular remedies before the test.
• Stop estrogen-containing medications: Estrogen-containing medications, including the contraceptive pill and hormone replacement therapy, should be stopped for six weeks before measuring serum cortisol levels.
• Follow the test instructions carefully: The most common cause of an abnormal test result is when instructions are not followed. Patients should read and follow the instructions provided by their healthcare provider.
• Collect urine samples (if applicable): For the standard low-dose method, patients may need to collect urine samples for 24-hour periods over three days to measure cortisol levels.

On the day of the test, patients should expect the following:

• Dexamethasone administration: Patients will receive dexamethasone, a synthetic glucocorticoid, which can be administered orally or through an IV.
• Blood draw: A healthcare provider will draw a blood sample to measure cortisol levels.
• Urine collection (if applicable): Patients may need a urine sample for free cortisol measurement for the standard low-dose method.

Patients must follow their healthcare provider's instructions and notify them if they have any concerns or questions about the test.

Dexamethasone Suppression Test procedure

The overnight dexamethasone suppression test (DST) is a diagnostic procedure used for assessing cortisol suppression, aiding in the differential diagnosis of conditions such as Cushing's disease. It involves administering a low dose of dexamethasone, a synthetic glucocorticoid, to observe cortisol response. Two variations exist: the late-night salivary cortisol test and the urinary-free cortisol test, designed to evaluate cortisol levels.

Procedure for overnight Dexamethasone Suppression Test

Late-night salivary cortisol

1. Baseline Measurement: Obtain a baseline cortisol measurement in the morning.
2. Dexamethasone administration: Administer 1 mg of dexamethasone at 11 p.m.
3. Post-dexamethasone measurement: Draw blood at 8 a.m. for cortisol level assessment.

Urinary free cortisol (3-day method)

1. Baseline measurement: Collect urine over three days for cortisol measurement.
2. Dexamethasone administration: Administer 1 mg of dexamethasone at 11 p.m.
3. Post-dexamethasone measurement: Continue urine collection, with cortisol levels assessed over the three days.

For the high-dose test, the procedure is similar, but with an elevated dexamethasone dose (8 mg) administered overnight. Additional measurements are taken to evaluate cortisol suppression, including plasma cortisol and late-night salivary cortisol.

Interpreting the results

The interpretation of the intravenous dexamethasone suppression test (DST) revolves around assessing cortisol levels pre- and post-administration of dexamethasone, a synthetic glucocorticoid. Dexamethasone typically reduces adrenocorticotropic hormone (ACTH) release from the pituitary gland, decreasing serum cortisol levels. This test is crucial in evaluating Cushing's syndrome (CS), specifically by gauging the suppression of endogenous cortisol secretion.

Critical considerations for interpretation include:

• Normal response: For the low-dose overnight method, a standard response involves a decline in cortisol levels after administering 1 mg of dexamethasone at 11 p.m. Blood is drawn the following morning at 8 a.m. for cortisol measurement. In the high-dose test, a normal response is observed when cortisol levels decrease after administering 8 mg of dexamethasone at 11 p.m., followed by a morning blood draw at 8 a.m.
• Abnormal response: An abnormal response may suggest excessive cortisol production. Specifically, an abnormal low-dose test response coupled with a normal high-dose test response may indicate overproduction of ACTH by a pituitary tumor.
• Diagnostic context: Healthcare professionals must interpret results in the context of the patient's clinical condition, considering additional diagnostic tests, such as pituitary MRI and other biochemical testing. Suspected Cushing syndrome, adrenal tumors, and cortisol-related disorders warrant thorough evaluation through screening tests.
• Dexamethasone administration: Intravenous dexamethasone is administered to evaluate cortisol suppression. The test helps discern between ACTH-dependent and ACTH-independent causes of Cushing syndrome.
• Cortisol measurement: Serum cortisol levels and plasma ACTH are pivotal in result interpretation.
• False test results: Factors like corticotropin-releasing hormone (CRH), cortisol-binding globulin (CBG), and glucocorticoid administration should be considered to prevent misinterpretation or false test results.
• Supraphysiological dose: High-dose dexamethasone administration suppresses cortisol production and can be instrumental in diagnosing Cushing disease.
• Pituitary tumor and ACTH production: Assessment of pituitary tumors, particularly their impact on ACTH production, is essential for accurate diagnosis.
• Negative feedback mechanism: Understanding the negative feedback loop involving dexamethasone, ACTH, and cortisol is crucial in interpreting results.
• Screening and sensitivity: The DST is part of screening tests for suspected Cushing syndrome, ensuring diagnostic sensitivity and reliable outcomes.

Other similar tests

Various alternative and complementary tests to the Dexamethasone Suppression Test (DST) are available to assess adrenal cortex function, ACTH secretion, and serum cortisol levels and diagnose conditions like Cushing's syndrome. These include:

• 24-hour urinary free cortisol test: This measures the quantity of cortisol excreted in urine over 24 hours, aiding in diagnosing Cushing's syndrome.
• Late-night salivary cortisol test: This assesses cortisol levels in saliva during nighttime, contributing to the diagnosis of Cushing's syndrome.
• ACTH stimulation test: Evaluates adrenal gland response to ACTH, aiding in diagnosing adrenal insufficiency and other cortisol-related conditions.
• CRH stimulation test: Assesses the pituitary gland's response to CRH, assisting in diagnosing conditions related to cortisol production, including Cushing's syndrome.
• Imaging tests: Techniques such as CT scans and MRI are employed to identify the cause of excessive cortisol production, such as tumors in the adrenal or pituitary glands.

When selecting a test, it is crucial to consider the individual patient's clinical condition and the healthcare provider's judgment. While the test is commonly used, it may not suit all patients.

Factors such as adrenal tumors, corticosteroid-binding globulin, and normal cortisol suppression levels need to be taken into account, and other tests like the overnight suppression test and blood tests measuring plasma cortisol levels should be considered based on the patient's unique circumstances.

Additionally, assessing too much cortisol, acth secretion, and the potential for pseudo-Cushing's syndrome are integral components of an effective diagnostic approach.

Follow-up care and next steps

After undergoing the Dexamethasone Suppression Test (DST), post-test care is contingent on the patient's clinical condition and results. The following measures may be recommended:

• Repeat testing: Should the initial results prove abnormal or inconclusive, healthcare providers may advise additional testing to validate the diagnosis and pinpoint the specific cause of the abnormality.
• Imaging studies: Depending on the patient's clinical profile, healthcare providers may suggest imaging tests such as CT scans or MRI to identify the source of elevated cortisol production, such as tumors in the adrenal or pituitary glands.
• Endocrine evaluation: A thorough evaluation of the endocrine system may be warranted for patients with a history of other endocrine disorders.
• Monitoring medication side effects: If the patient is on medications that could influence test results, healthcare providers may closely monitor for potential side effects and adjust the medication regimen accordingly.
• Titrating down or discontinuing steroid therapy: In cases where patients are on high doses of steroids, healthcare providers may gradually reduce the dosage or discontinue therapy based on the patient's clinical condition and treatment response