Should serum ACTH also be measured at the same time as the cortisol sample during the high-dose dexamethasone suppression test?

ACTH Measurement During Dexamethasone Suppression Testing

ACTH measurement is not routinely required during the low-dose dexamethasone suppression test (DST) itself, but should be measured separately to determine the etiology of confirmed hypercortisolism—specifically to distinguish adrenal from pituitary/ectopic sources of Cushing syndrome.

When to Measure ACTH

For Initial Screening (Low-Dose DST)

• Do not measure ACTH during the overnight 1-mg DST or 48-hour low-dose DST 1
• These tests assess cortisol suppression only—measuring cortisol at 0800-0900h after dexamethasone administration
• The goal is to detect autonomous cortisol production, not to determine its source

For Determining Etiology (After Positive Screening)

Measure basal morning ACTH (0800-0900h) separately when:

• Hypercortisolism is confirmed by positive screening tests
• You need to distinguish ACTH-dependent from ACTH-independent causes

Key diagnostic thresholds:

• ACTH <1.1 pmol/L (<5 ng/L): Suggests adrenal source (ACTH-independent) 2
• ACTH ≥1.1 pmol/L (≥5 ng/L): Suggests pituitary or ectopic source (ACTH-dependent) 3
• ACTH >6.4 pmol/L (>29 ng/L): 70% sensitivity and 100% specificity for Cushing disease 3

Special Circumstances Where ACTH May Be Helpful

During High-Dose DST for Differential Diagnosis

Recent evidence suggests measuring ACTH during high-dose DST may provide additional diagnostic value 4, 5:

• One study found that measuring both cortisol and ACTH during the standard high-dose DST improved diagnostic accuracy
• However, the low-dose DST combined with CRH testing provides equivalent or superior diagnostic information without requiring high-dose dexamethasone 5

In Adrenal Incidentalomas

Measure basal ACTH when post-DST cortisol >1.8 μg/dL (>50 nmol/L):

• Low/suppressed ACTH confirms autonomous cortisol secretion from the adrenal mass 2
• This distinguishes mild autonomous cortisol secretion (MACS) from other causes of inadequate suppression
• Recent data show ACTH measurement after overnight DST may help identify false-positive results 6, 7

Novel approach (requires validation):

• ACTH measured after overnight DST <0.6 pmol/L suggests true HPA axis suppression 6
• ACTH ≥0.6 pmol/L after DST may indicate inadequate dexamethasone absorption or other confounders 6

In Patients on Oral Contraceptives or with CKD

ACTH measurement significantly improves specificity 7:

• Oral contraceptive users have falsely elevated cortisol due to increased cortisol-binding globulin
• Post-DST ACTH (cutoff 10 pg/mL) showed 100% specificity in these populations
• Standard cortisol-only DST had poor specificity (55% in COCP users, 17% in CKD patients)

Practical Algorithm

Step 1: Screen for Hypercortisolism

Perform overnight 1-mg DST (or 48-hour low-dose DST):

• Give 1 mg dexamethasone at 2300-2400h
• Measure cortisol only at 0800-0900h
• Cutoff: >1.8 μg/dL (>50 nmol/L) suggests hypercortisolism

Step 2: Confirm Hypercortisolism

Use additional tests (24-hour UFC, late-night salivary cortisol) 2, 1

Step 3: Determine Etiology

Measure basal morning ACTH (separate from DST):

• If ACTH suppressed (<1.1 pmol/L): Adrenal source → proceed to adrenal imaging
• If ACTH normal/elevated (≥1.1 pmol/L): Pituitary or ectopic source → proceed to CRH testing and pituitary imaging

Step 4: Consider ACTH During DST Only If:

• Patient on oral contraceptives or has CKD (measure post-DST ACTH to improve specificity) 7
• Borderline results and need to distinguish inadequate dexamethasone absorption from true hypercortisolism 6
• Research protocol or validation of novel diagnostic approaches

Important Caveats

Why ACTH is not routinely measured during low-dose DST:

• Morning ACTH represents only basal, biologically inactive secretion that cannot be further suppressed by dexamethasone 8
• The low-dose DST tests cortisol suppression, not ACTH dynamics
• ACTH measurement adds cost without improving diagnostic accuracy for initial screening 1

False-positive DST results (where ACTH measurement helps):

• Inadequate dexamethasone absorption (measure dexamethasone levels if available) 9
• Increased cortisol-binding globulin (oral contraceptives, pregnancy) 7
• Drug interactions affecting dexamethasone metabolism 1, 10

The high-dose DST is largely obsolete:

• Low-dose DST combined with CRH testing provides equivalent diagnostic information 5
• No need to perform high-dose DST routinely in modern practice