Serum Cortisol Measurement: Primary Diagnostic Applications
Serum cortisol is primarily used to diagnose Cushing's syndrome (cortisol excess) and adrenal insufficiency/Addison's disease (cortisol deficiency). 1, 2
Primary Disease Applications
Cushing's Syndrome (Cortisol Excess)
Serum cortisol measurement is a cornerstone diagnostic test for Cushing's syndrome, specifically through the overnight 1-mg dexamethasone suppression test (DST). 1 In this test:
- Normal response: Serum cortisol <1.8 μg/dL (50 nmol/L) at 0800h after 1 mg dexamethasone given at 2300h-midnight
- Abnormal response: Failure to suppress below this threshold indicates autonomous cortisol production
- At higher cutoffs (>5 μg/dL/138 nmol/L), the test identifies overt Cushing's syndrome with higher specificity but reduced sensitivity 1
The DST has >90% sensitivity for detecting Cushing's syndrome, making it one of the most sensitive screening tests available. 1
Primary Adrenal Insufficiency (Addison's Disease)
Paired measurement of serum cortisol and plasma ACTH is the diagnostic test of choice for primary adrenal insufficiency. 2 Key diagnostic thresholds include:
- Serum cortisol <250 nmol/L with elevated ACTH in acute illness is diagnostic of primary adrenal insufficiency 2
- Serum cortisol <400 nmol/L with elevated ACTH in acute illness raises strong suspicion 2
- In equivocal cases, a synacthen-stimulated peak cortisol <500 nmol/L confirms the diagnosis 2
For screening purposes, a morning serum cortisol <275 nmol/L identifies subnormal adrenal function with 96.2% sensitivity, while an afternoon cortisol <250 nmol/L achieves 96.1% sensitivity. 3 These single measurements can significantly reduce the need for dynamic testing in outpatient settings.
Secondary Applications
Adrenal Incidentalomas
Serum cortisol after DST helps evaluate dysregulated cortisol secretion from adrenal masses:
- Cortisol <1.8 μg/dL excludes autonomous cortisol production
- Cortisol >5 μg/dL identifies patients with overt Cushing's syndrome from the incidentaloma 1
Hypothalamic-Pituitary-Adrenal Axis Disorders
Basal morning serum cortisol serves as a first-line screening test, with cortisol >285 nmol/L having 100% sensitivity for excluding adrenal insufficiency, while cortisol <98 nmol/L has 100% specificity for confirming it. 4
Critical Interpretation Caveats
Factors Causing False Results
False positive DST results (inappropriately high cortisol) occur with:
- CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort)
- Increased corticosteroid-binding globulin from oral estrogens, pregnancy, or chronic hepatitis
- Malabsorption conditions (celiac disease, chronic diarrhea) 1
False negative DST results (inappropriately low cortisol) occur with:
- Medications inhibiting dexamethasone metabolism (fluoxetine, cimetidine, diltiazem)
- Decreased binding proteins (nephrotic syndrome) 1
Measuring dexamethasone levels concomitantly with cortisol can reduce false-positive results. 1
Assay-Specific Issues
Immunoassays show considerable inter- and intra-assay variation with matrix effects in pregnancy (underrecovery) and cross-reactivity with synthetic steroids like metyrapone and prednisolone (overestimation). 5 LC-MS/MS offers superior specificity and accuracy compared to immunoassays, though cortisol cutoffs require validation for these newer methods. 6