Laboratory Evaluation for Hypercortisolism in Adrenal Nodules
For patients with adrenal nodules, start with the 1-mg overnight dexamethasone suppression test (DST) as the simplest and most practical screening test for autonomous cortisol secretion. 1
Initial Screening Approach
The biochemical workup follows a two-step process: first confirming hypercortisolism, then determining its etiology (ACTH-dependent vs. ACTH-independent).
Step 1: Screen for Hypercortisolism
Primary screening test:
- 1-mg overnight DST: Give 1 mg dexamethasone at 11 PM, measure fasting serum cortisol at 8-9 AM the next morning 1
- Cortisol <50 nmol/L (<1.8 μg/dL): Excludes hypercortisolism
- Cortisol 51-138 nmol/L (1.8-5.0 μg/dL): Possible mild autonomous cortisol secretion (MACS)—proceed with confirmatory testing
- Cortisol >138 nmol/L (>5.0 μg/dL): Evidence of cortisol hypersecretion 2
If clinical suspicion is high (hypertension, obesity, diabetes, osteoporosis), use all three complementary tests since they target different pathophysiologic aspects 1:
Late-night salivary cortisol (2 samples): Sensitivity and specificity 90-95% 1
- Tests loss of normal circadian rhythm
- Patient collects saliva at bedtime at home
- Based on local assay cut-offs
24-hour urinary free cortisol (UFC) (collect for 3 consecutive days): 1
- >4× upper limit of normal: Diagnostic of Cushing syndrome
- 2-4× upper limit: May represent pseudo-Cushing (anxiety, depression, alcoholism, obesity)—needs confirmatory testing
- Reflects integrated cortisol production over 24 hours
- Not affected by cortisol-binding globulin changes
Step 2: Determine Etiology
Once hypercortisolism is confirmed, measure plasma ACTH (morning, 8-9 AM) to distinguish adrenal from pituitary/ectopic sources 1:
- ACTH low or suppressed (<5 ng/L or <1.1 pmol/L): Confirms adrenal origin (ACTH-independent Cushing syndrome from the adrenal nodule) 3
- ACTH normal or elevated (>5 ng/L or >1.1 pmol/L): Indicates ACTH-dependent Cushing (pituitary or ectopic source, NOT from the adrenal nodule) 3
For borderline ACTH levels, consider CRH stimulation test: lack of ACTH response distinguishes ACTH-independent from ACTH-dependent Cushing syndrome 1.
Critical Pitfalls to Avoid
DST false-positives occur with:
- CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort)—increase dexamethasone metabolism 4
- Oral estrogens, pregnancy, chronic active hepatitis—increase cortisol-binding globulin 4
- Solution: Measure dexamethasone levels concomitantly with cortisol 4
DST false-negatives occur with:
- Fluoxetine, cimetidine, diltiazem—inhibit dexamethasone metabolism 4
- Nephrotic syndrome—decreases cortisol-binding globulin 4
UFC limitations:
- Requires complete 24-hour collection with appropriate volume
- Less reliable with renal impairment (CrCl <60 mL/min) or polyuria (>5 L/24h)—use late-night salivary cortisol instead 4
- Affected by sex, BMI, age, urinary volume, and sodium intake 4
Salivary cortisol issues:
- Requires adequate soaking of collection device
- May miss cyclic Cushing syndrome 5
- Disrupted circadian rhythm (shift workers, jet lag, severe insomnia) can falsely lower results 6
Clinical Context
More than 5% of adrenal incidentalomas have subclinical Cushing syndrome (now termed MACS) 1. This represents subtle autonomous cortisol production without overt Cushingoid features but is associated with higher rates of hypertension, obesity, insulin resistance, dyslipidemia, and osteoporosis 1.
The simplest approach: Start with 1-mg overnight DST. If abnormal, add ACTH to confirm adrenal origin. If clinical suspicion remains high despite normal DST, add late-night salivary cortisol and 24-hour UFC since no single test has 100% sensitivity 1.