What does a cortisol level of 1.2 after a dexamethasone (DEX) suppression test indicate?

Interpretation of Cortisol 1.2 after Dexamethasone Suppression Test

A cortisol level of 1.2 after a dexamethasone suppression test indicates normal suppression and effectively rules out Cushing's syndrome. 1, 2

Normal Values and Interpretation

• The Endocrine Society recommends that a normal response to the overnight 1-mg dexamethasone suppression test (DST) is a cortisol level <1.8 μg/dL (50 nmol/L) 1, 2
• A cortisol value of 1.2 falls below this cutoff, indicating appropriate hypothalamic-pituitary-adrenal axis suppression 2
• This result strongly predicts the absence of Cushing's syndrome, as it demonstrates the ability of exogenous glucocorticoids to suppress endogenous cortisol production 1
• Earlier studies suggested a cutoff of <2 μg/dL rather than the previously accepted <5 μg/dL, making your result of 1.2 clearly within normal limits 3

Clinical Significance

• The overnight DST has high sensitivity but moderate specificity for diagnosing hypercortisolism, making it particularly useful for ruling out Cushing's syndrome 1, 4
• A normal result like 1.2 effectively excludes autonomous cortisol production that would be seen in conditions such as Cushing's syndrome or autonomous cortisol secretion from adrenal incidentalomas 1, 2
• This normal suppression indicates intact negative feedback in the hypothalamic-pituitary-adrenal axis 2

Potential Confounding Factors

• Certain medications that affect CYP3A4 (which metabolizes dexamethasone) can influence test results 1, 2
• Oral estrogen therapy can increase cortisol-binding globulin levels, potentially leading to false positive results, but this is not a concern with your normal result 1, 2
• Rapid absorption/malabsorption of dexamethasone can affect interpretation, but simultaneous measurement of dexamethasone levels can help confirm adequate absorption 4, 5
• In some cases, measuring free cortisol rather than total cortisol may provide additional diagnostic clarity, especially in patients with altered binding proteins 6

Next Steps

• Given the normal suppression result, no further testing for Cushing's syndrome is warranted unless there are strong clinical features suggesting the condition 1, 2
• If the test was performed for evaluation of an adrenal incidentaloma, this result indicates absence of autonomous cortisol secretion 1
• If clinical suspicion for Cushing's syndrome remains high despite this normal result, alternative tests such as late-night salivary cortisol or 24-hour urinary free cortisol could be considered 2

Important Considerations

• The DST evaluates only cortisol suppression; ACTH levels should not be measured during this test as it is not part of the standard protocol 2
• A normal DST result does not exclude all forms of adrenal dysfunction, only hypercortisolism 1, 2
• The test has highest diagnostic accuracy when performed correctly with dexamethasone administered between 11 PM and midnight and cortisol measured at 8 AM the following morning 2