Interpretation of DST Cortisol Level of 0.9 μg/dL
A cortisol level of 0.9 μg/dL (24.8 nmol/L) after a dexamethasone suppression test is normal and effectively rules out autonomous cortisol secretion or Cushing's syndrome. 1, 2
Normal Reference Range
Normal cortisol suppression after the 1 mg overnight DST should be <1.8 μg/dL (<50 nmol/L), making your result of 0.9 μg/dL well within the normal range. 1, 2
Values between 1.8-5.0 μg/dL (50-138 nmol/L) indicate possible autonomous cortisol secretion and require further evaluation, while values >5.0 μg/dL (>138 nmol/L) provide strong evidence of cortisol hypersecretion. 1, 2
Your result of 0.9 μg/dL demonstrates intact negative feedback of the hypothalamic-pituitary-adrenal axis and normal suppression by exogenous glucocorticoid. 1
Clinical Implications
This result excludes Cushing's syndrome and autonomous cortisol secretion with high confidence. 1, 2
No further biochemical testing for hypercortisolism is indicated based on this result alone. 1
If this test was performed as part of an adrenal incidentaloma workup, the adrenal mass can be classified as non-functioning from a cortisol perspective. 1
Important Caveats
Ensure adequate dexamethasone absorption occurred - if there is clinical suspicion that the patient did not take the medication properly or has malabsorption, measuring concomitant dexamethasone levels (should be ≥0.1 mcg/dL) can confirm adequate drug exposure. 3
CYP3A4 inducers (phenytoin, rifampin, carbamazepine, barbiturates) can accelerate dexamethasone metabolism and cause falsely reassuring results, though this would typically result in higher cortisol values, not lower ones like 0.9 μg/dL. 2
Cyclic Cushing's syndrome is a rare pitfall where patients alternate between periods of normal and elevated cortisol secretion - if clinical suspicion remains high despite this normal result, repeat testing during symptomatic periods may be warranted. 2, 4
This test does not assess adrenal reserve or rule out adrenal insufficiency - it only evaluates for cortisol excess. 2