Dexamethasone Suppression Test (DST) - Variants and Differential Diagnosis

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Katzung's Basic and Clinical Pharmacology, 16th Edition
• Schwartz's Principles of Surgery, 11th Edition

The Three Main Variants

1. Overnight 1 mg DST (Low-Dose Screening Test)

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Schwartz's Principles of Surgery, 11th Edition

2. Standard 2-Day Low-Dose DST (Liddle Test - Confirmatory)

• UFC suppressed to < 50% of baseline → Cushing syndrome ruled out
• UFC not suppressed (≥ 50% of baseline) → Cushing syndrome confirmed

3. High-Dose DST (Differential Diagnostic Test)

• Standard: 2 mg dexamethasone every 6 hours for 48 hours (total 8 mg), with 24-hour UFC collected on day 2
• Overnight: Single dose of 8 mg at 11 PM, plasma cortisol measured at 8 AM next morning

• In Cushing disease (pituitary adenoma): the adenoma retains some glucocorticoid receptor sensitivity - it has a raised set-point, not complete resistance. High doses of dexamethasone can still eventually suppress it.
• In ectopic ACTH (e.g., small cell lung cancer, bronchial carcinoid, thymoma): the tumor secretes ACTH autonomously, completely outside pituitary control, and is not suppressible by any dose of dexamethasone.
• In adrenal tumors: cortisol secretion is autonomous and ACTH-independent; ACTH is already suppressed by the high cortisol, and dexamethasone has no additional effect.
• Scott-Brown's Otorhinolaryngology, Vol 1
• Robbins, Cotran & Kumar Pathologic Basis of Disease
• Schwartz's Principles of Surgery, 11th Edition
• Katzung's Basic and Clinical Pharmacology, 16th Edition

4. Combined Dexamethasone-CRH Test (Pseudo-Cushing Differentiation)

• Plasma cortisol > 1.4 µg/dL (38 nmol/L) at 15 or 30 minutes post-CRH → true Cushing syndrome (high specificity)
• Suppressed cortisol post-CRH → pseudo-Cushing (e.g., depression, alcoholism, obesity)

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Katzung's Basic and Clinical Pharmacology, 16th Edition

Diagnostic Algorithm

1. Screening (overnight 1 mg DST + UFC): if suppressed → Cushing ruled out. If not suppressed →
2. High-dose DST + UFC:
   • UFC < 50% of baseline → Cushing disease (pituitary source)
   • UFC ≥ 50% of baseline → measure plasma ACTH:
     • ACTH < 10 pg/mL → adrenocortical tumor (primary adrenal)
     • ACTH > 50 pg/mL → pituitary MRI
       • Positive MRI → Cushing disease
       • Negative MRI → ectopic ACTH/CRH syndrome (plasma ACTH may exceed 300 pg/mL) → consider bilateral inferior petrosal sinus sampling (BIPSS) as gold standard

Inferior Petrosal Sinus Sampling (BIPSS) - The Gold Standard

Summary Table

The Physiological Basis: Diurnal Variation

• Campbell-Walsh-Wein Urology, 3-Volume Set

Midnight Serum Cortisol

• In a sleeping patient (ideally inpatient): cortisol > 1.8 µg/dL is abnormal (very sensitive threshold)
• In an awake patient: cortisol > 7.5 µg/dL is used (higher threshold because wakefulness itself raises cortisol via stress)

• Campbell-Walsh-Wein Urology
• Textbook of Family Medicine, 9th Edition

Late-Night Salivary Cortisol (LNSC)

• Normal: < 0.14 µg/dL (< 4 nmol/L) at midnight
• Abnormal (suggestive of Cushing): > 0.14 µg/dL on two separate occasions

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Smith and Tanagho's General Urology, 19th Edition

How They Fit Into the Screening Algorithm

• Tietz Textbook of Laboratory Medicine, 7th Edition
• Quick Compendium of Clinical Pathology, 5th Edition

Key Clinical Points

1. All screening tests - DST, UFC, and midnight cortisol - tell you only that Cushing syndrome is present (or absent). None of them alone distinguish the type (pituitary vs. adrenal vs. ectopic). That differentiation requires plasma ACTH and the high-dose DST as discussed previously.
2. Cyclic Cushing syndrome is an important pitfall: cortisol secretion can be intermittently normal and intermittently elevated. UFC and salivary cortisol, which must be repeated on multiple occasions, tend to catch this better than a single overnight DST.
3. Subclinical Cushing syndrome (autonomous cortisol secretion without overt clinical features, often found in adrenal incidentalomas) - the UFC is specifically not recommended by the Endocrine Society for this indication because it lacks sufficient sensitivity. The 1 mg overnight DST and LNSC are preferred for this population.

• Campbell-Walsh-Wein Urology, 3-Volume Set
• Current Surgical Therapy, 14th Edition

Answer: A. MRI Brain (Pituitary MRI)

The Reasoning

1. Cushing's disease - pituitary corticotrope adenoma (~80% of ACTH-dependent cases)
2. Ectopic ACTH syndrome - non-pituitary tumor secreting ACTH (e.g., small-cell lung cancer, bronchial carcinoid, thymoma)

"For ACTH-dependent cortisol excess, a magnetic resonance image (MRI) of the pituitary is the investigation of choice..."

• Harrison's Principles of Internal Medicine, 22nd Edition (2025)

"Pituitary MRI should be obtained routinely in patients with ACTH-dependent disease to exclude a macroadenoma or abnormal anatomy before petrosal sinus sampling or surgery."

Why Not the Other Options?

The Critical Caveat

• MRI is negative, or
• MRI shows a lesion < 6 mm (non-diagnostic), or
• Biochemical tests (high-dose DST, CRH test) give equivocal results

Sequence Summary

Confirmed Cushing's (screening positive)
        ↓
Plasma ACTH measurement
        ↓
ACTH elevated (≥15 pg/mL) → ACTH-dependent Cushing's
        ↓
→ MRI PITUITARY ← (investigation of choice)
        ↓
If positive (lesion ≥6-9 mm) + supportive biochemistry → Cushing's disease → surgery
If negative/equivocal → High-dose DST + CRH test + Desmopressin test
        ↓
If still equivocal → IPSS (gold standard)