Dexamethasone suppression test for cortisol

Overview

The Dexamethasone Suppression Test (DST) is a specialized diagnostic test used to evaluate the hypothalamic–pituitary–adrenal (HPA) axis and assess whether cortisol secretion from the adrenal gland can be appropriately suppressed. Dexamethasone, a synthetic glucocorticoid, works through a negative feedback mechanism, suppressing ACTH production and thereby reducing cortisol levels in healthy individuals.

This test is essential for diagnosing and differentiating disorders related to cortisol excess or deficiency, including Cushing syndrome, adrenal insufficiency, and abnormalities in ACTH production. Several forms of the test exist:

1. Low-dose DST (1 mg) for initial screening of Cushing syndrome
2. High-dose DST (8 mg) for distinguishing pituitary Cushing disease from ectopic ACTH production
3. Intravenous DST for patients with suspected malabsorption or poor medication compliance
4. Dexamethasone–CRH Test for differentiating true Cushing disease from pseudo-Cushing states

The test uses electrochemiluminescence immunoassay for accurate cortisol measurement. Proper specimen handling—keeping blood at room temperature, separating serum/plasma quickly, and refrigerating samples—is crucial for reliable results.

Symptoms

While DST itself does not produce symptoms, the PDF highlights clinical presentations that lead to DST ordering. These symptoms represent suspected cortisol abnormalities that require evaluation.

Symptoms Suggestive of Hypercortisolism (Cushing Syndrome):

1. Unexplained weight gain
2. Changes in mood, irritability, or depression
3. Hypertension
4. Menstrual irregularity
5. Muscle weakness
6. Skin changes, including easy bruising
7. Uncontrolled blood sugar

Symptoms Suggestive of Adrenal Insufficiency:

1. Fatigue
2. Weight loss
3. Low blood pressure
4. Nausea or vomiting
5. Muscle pain

Symptoms Indicating Abnormal ACTH Sources:

1. Rapid worsening of symptoms
2. Severe metabolic disturbances
3. Features inconsistent with typical pituitary disease

These symptoms help clinicians determine which DST protocol—low dose, high dose, IV, or Dex-CRH—is appropriate.

Causes

According to the PDF, the DST identifies disorders in which cortisol does not suppress normally. Causes of abnormal test results include endocrine diseases, tumors, medication use, and physiologic stress.

Causes of Increased Cortisol Levels (Failure to Suppress):

1. Cushing syndrome
2. Pituitary tumor (Cushing disease)
3. Adrenal tumors – benign or malignant
4. Ectopic ACTH production
5. Congenital adrenal hyperplasia
6. Polycystic ovary syndrome (PCOS)

Causes of Decreased Cortisol Levels:

1. Addison’s disease
2. Hypopituitarism

Interfering Drugs (May Alter Results):

1. Anti-seizure medications
2. Barbiturates
3. Steroid medications (hydrocortisone, prednisone)
4. Estrogen
5. Oral contraceptives
6. Diuretics

False-Positive Causes (Cortisol appears elevated):

1. Stress
2. Weight loss
3. Alcohol withdrawal
4. Drugs: Diphenylhydantoin, Phenobarbital, Rifampicin, Carbamazepine, Lithium

False-Negative Causes (Cortisol appears low when it should not):

1. Liver disease
2. Benzodiazepines
3. Methadone
4. Ketoconazole
5. Indomethacin

Understanding these causes ensures accurate interpretation of DST results.

Risk Factors

The PDF identifies several patient-related and medical factors that increase the likelihood of abnormal cortisol secretion or influence DST accuracy.

Risk Factors for Abnormal Cortisol Levels:

1. Pituitary tumors
2. Adrenal tumors
3. Ectopic ACTH-secreting tumors
4. Autoimmune adrenal destruction (Addison’s)
5. Chronic stress or depression-related cortisol elevation
6. Congenital adrenal hyperplasia
7. PCOS

Risk Factors That Impact Test Interpretation:

1. Taking corticosteroids or estrogen-containing medications
2. Uncontrolled diabetes
3. Recent surgery or trauma
4. Severe mental health disorders
5. Active systemic infections
6. Pregnancy and breastfeeding (require caution)
7. Poor compliance with oral medication in DST
8. Malabsorption affecting dexamethasone uptake

All these factors must be considered before conducting the test to avoid misleading results.

Prevention

Prevention strategies in the PDF focus on avoiding inaccurate test results, ensuring safe sample handling, and identifying conditions where the test is contraindicated.

Clinical Prevention Measures:

1. Avoid DST in individuals with hypersensitivity to dexamethasone.
2. Delay testing in patients with serious infections, uncontrolled diabetes, or shortly after surgery/trauma.
3. Use DST with caution in pregnancy or breastfeeding.
4. Review current medications that may interfere with cortisol levels.

Testing-Related Prevention:

1. Collect 3 mL blood in a plain red-capped, green-capped (heparin), or lavender-capped (EDTA) tube.
2. Allow blood to clot at room temperature and separate serum/plasma within 2 hours.
3. Store samples refrigerated; stability is:
   1. 24 hours at room temperature
   2. 4 days refrigerated
   3. 12 months frozen
4. Avoid repeated freeze–thaw cycles.
5. Perform IV DST when oral compliance is uncertain.
6. Use Dexamethasone-CRH test when differentiating Cushing disease vs ectopic ACTH.