The 1 mg dexamethasone suppression test is a diagnostic procedure used to evaluate the body’s regulation of cortisol, a hormone produced by the adrenal glands. This test helps healthcare providers understand how the body’s hormonal system responds to a synthetic glucocorticoid, providing insight into potential imbalances in cortisol production. It serves as an initial assessment tool when cortisol dysregulation is suspected.
What the Test Screens For
The 1 mg dexamethasone suppression test primarily screens for Cushing’s syndrome, a condition characterized by prolonged exposure to excessive cortisol levels in the body. Cortisol, often referred to as the “stress hormone,” plays a role in metabolism, immune function, and the body’s response to stress. Normally, cortisol levels follow a daily rhythm, peaking in the morning and declining throughout the day.
Cortisol production is regulated by a complex feedback system involving the hypothalamus, pituitary gland, and adrenal glands, known as the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the pituitary gland to produce adrenocorticotropic hormone (ACTH). ACTH then travels to the adrenal glands, prompting them to release cortisol.
In a healthy individual, elevated cortisol levels provide negative feedback to the hypothalamus and pituitary, signaling them to reduce CRH and ACTH production, thereby lowering cortisol levels. In Cushing’s syndrome, this feedback mechanism is disrupted, leading to uncontrolled cortisol secretion. This disruption can stem from a pituitary tumor overproducing ACTH (Cushing’s disease), or less commonly, an adrenal gland tumor directly producing excess cortisol, or a non-pituitary tumor elsewhere in the body secreting ACTH.
How the Test is Performed
The 1 mg overnight dexamethasone suppression test is a simple outpatient procedure. Patients take a single 1 milligram (mg) dose of dexamethasone orally in the late evening, typically between 11:00 PM and midnight. Dexamethasone is a synthetic steroid that mimics cortisol and is used to assess the body’s ability to suppress its own cortisol production.
The following morning, usually between 8:00 AM and 9:00 AM, a blood sample is collected. The blood sample is then analyzed to measure the circulating cortisol levels. The timing of the blood draw is specific because cortisol naturally peaks in the early morning, and the test aims to see if dexamethasone has successfully suppressed this usual morning surge.
Understanding the Results
Interpreting the results of the 1 mg dexamethasone suppression test involves evaluating the morning cortisol level after dexamethasone administration. In a healthy individual, the 1 mg dose of dexamethasone should suppress ACTH secretion from the pituitary gland, leading to a significant decrease in the body’s own cortisol production. A normal response is typically indicated by a morning serum cortisol level less than 1.8 micrograms per deciliter (mcg/dL), or 50 nanomoles per liter (nmol/L). This low cortisol level suggests that the HPA axis is functioning properly and can be appropriately suppressed.
Conversely, an abnormal result occurs when the morning cortisol level remains elevated, failing to suppress below the threshold of 1.8 mcg/dL (or 50 nmol/L). Cortisol levels above this threshold, particularly those greater than 5.0 mcg/dL (138 nmol/L), suggest a lack of proper suppression, which is indicative of potential autonomous cortisol secretion or Cushing’s syndrome. If the cortisol level is between 1.8 mcg/dL and 5.0 mcg/dL, it may suggest possible autonomous cortisol secretion. An abnormal test result warrants further diagnostic investigations to confirm the presence of Cushing’s syndrome and determine its underlying cause.
Factors Influencing Test Accuracy
Several factors can influence the accuracy of the 1 mg dexamethasone suppression test, potentially leading to misleading results. Medications can interfere with cortisol metabolism or the action of dexamethasone. For instance, oral contraceptives containing estrogen can increase levels of cortisol-binding globulin, which can lead to falsely elevated total cortisol measurements. It is often recommended that women discontinue oral contraceptives for about six weeks before the test to avoid this interference.
Other medications, such as anticonvulsants (phenytoin, phenobarbitone, and carbamazepine) or antibiotics (rifampicin), can induce the CYP3A4 enzyme in the liver, which metabolizes dexamethasone. This can cause dexamethasone to be cleared too quickly, resulting in an inappropriately low dexamethasone level and a false-positive cortisol reading, making it appear as though cortisol suppression did not occur. Conversely, drugs that inhibit CYP3A4, such as itraconazole or ritonavir, can decrease dexamethasone clearance, potentially leading to false-negative results. To improve accuracy, some laboratories may measure dexamethasone levels alongside cortisol to confirm adequate absorption and metabolism.
Beyond medications, physiological and psychological conditions can also affect test results. Acute illness, significant stress, severe obesity, and depression can lead to elevated cortisol levels, mimicking Cushing’s syndrome and causing false-positive results. Alcohol abuse and rapid weight loss are also known to interfere with test accuracy. Because of these potential interferences, it is important to inform the healthcare provider about all current medications, recent illnesses, and any other medical conditions before undergoing the test to ensure proper interpretation of the results.