Cushing questions love to feel “obvious” until the answer choices force you to separate ACTH-dependent vs ACTH-independent causes and decide what happens to cortisol, ACTH, and the adrenal glands. The trick is that the vignette rarely asks you to diagnose “Cushing” in general—it asks you to pinpoint where the problem is and what the next test/result will be.
The Vignette (Classic Q-bank Style)
A 34-year-old woman presents with progressive weight gain, proximal muscle weakness, easy bruising, and new-onset hypertension. Exam shows facial rounding, dorsocervical fat pad, and wide violaceous abdominal striae. Labs show hyperglycemia and hypokalemic metabolic alkalosis. A 24-hour urinary free cortisol is elevated on two separate collections. An overnight 1 mg dexamethasone suppression test fails to suppress cortisol. Plasma ACTH is elevated. A high-dose dexamethasone suppression test shows a >50% decrease in cortisol.
Question: What is the most likely diagnosis?
Correct answer: Cushing disease (pituitary ACTH-secreting adenoma)
Step 1: Lock In “True Cushing Syndrome”
Before localization, confirm hypercortisolism:
Screening tests (know these cold):
- 24-hour urinary free cortisol (elevated)
- Late-night salivary cortisol (elevated; loss of diurnal variation)
- Low-dose dexamethasone suppression test (no suppression)
If screening is positive, then you localize by measuring ACTH.
Why the Correct Answer Is Cushing Disease (Not Just “Syndrome”)
Key logic chain
- Cortisol high and not suppressed by low-dose dex → true Cushing syndrome.
- ACTH elevated → ACTH-dependent Cushing syndrome.
- High-dose dex suppresses cortisol by >50% → pituitary retains some feedback responsiveness → Cushing disease.
What “high-dose dex suppression” is really testing
Pituitary adenomas often partially suppress with high-dose dexamethasone, while ectopic ACTH tumors typically do not.
High-yield: Cushing disease = pituitary ACTH adenoma (ACTH-dependent)
Cushing syndrome = umbrella term for any cause of pathologic hypercortisolism.
The High-Yield Pattern Table (Answer Choice Sorting Tool)
| Cause | ACTH | Cortisol | Low-dose dex | High-dose dex | Adrenal imaging | Next best localization |
|---|---|---|---|---|---|---|
| Cushing disease (pituitary ACTH adenoma) | ↑ | ↑ | No suppress | Suppresses (>50%) | Bilateral adrenal hyperplasia | Pituitary MRI ± inferior petrosal sinus sampling |
| Ectopic ACTH (e.g., small cell lung carcinoma) | ↑↑ | ↑↑ | No suppress | No suppress | Bilateral adrenal hyperplasia | CT chest/abdomen, consider octreotide scan |
| Adrenal adenoma/carcinoma (ACTH-independent) | ↓ | ↑ | No suppress | No suppress | Unilateral adrenal mass | CT/MRI adrenal |
| Exogenous glucocorticoids | ↓ | ↓ (endogenous) | Already suppressed | Already suppressed | Bilateral adrenal atrophy | Med history (inhaled/topical/oral) |
Now, Why Every Distractor Is Wrong (and What It Would Look Like)
Below are the most common answer choices in this exact question family—and how to eliminate them fast.
Distractor 1: Ectopic ACTH production (Small cell lung carcinoma)
Why it’s tempting: ACTH is elevated and patient looks very Cushingoid.
Why it’s wrong here:
- Ectopic ACTH does not suppress with high-dose dex.
- Often has very high ACTH and severe hypokalemia due to cortisol’s mineralocorticoid activity at high levels (can still occur in Cushing disease, but classically more dramatic in ectopic ACTH).
What you’d expect instead:
- High-dose dex: no suppression
- Symptoms suggesting malignancy: weight loss, smoking history, cough/hemoptysis
- Next step: CT chest (small cell), consider other ectopic sources (bronchial carcinoid, pancreatic NET)
Distractor 2: Adrenal adenoma (ACTH-independent Cushing syndrome)
Why it’s tempting: “Adrenal disorder topic” primes you to pick an adrenal lesion.
Why it’s wrong here:
- ACTH is elevated in the stem; adrenal tumors cause low ACTH via negative feedback.
What you’d expect instead:
- ACTH low
- Imaging: unilateral adrenal mass
- Contralateral adrenal tends to be atrophic (suppressed)
- Treatment: surgical resection (adenoma) ± oncology workup (carcinoma)
Distractor 3: Exogenous glucocorticoid use
Why it’s tempting: Most common cause of Cushing syndrome overall.
Why it’s wrong here:
- Exogenous steroids suppress ACTH, so ACTH should be low.
- Endogenous cortisol production is low; depending on assay, measured serum cortisol may be low/normal, but clinical picture is Cushingoid.
What you’d expect instead:
- ACTH low
- Bilateral adrenal atrophy
- History clues: chronic oral prednisone, frequent steroid bursts, high-dose inhaled steroids, topical steroid overuse
USMLE pearl: Always ask about inhaled (asthma/COPD) and topical steroids—patients forget these “count.”
Distractor 4: Primary adrenal hyperplasia / Macronodular adrenal disease
Why it’s tempting: Still an adrenal cause of cortisol excess.
Why it’s wrong here:
- These are typically ACTH-independent → ACTH low.
- High-dose dex will not reliably suppress because the problem is in the adrenal gland.
What you’d expect instead:
- Low ACTH
- Imaging: bilateral nodularity/enlargement (varies by subtype)
- Workup leans on adrenal imaging and endocrine referral
Distractor 5: Pituitary prolactinoma or nonfunctioning pituitary adenoma
Why it’s tempting: You see “pituitary” and pick the most common pituitary tumor.
Why it’s wrong here:
- The hormonal pattern specifically localizes to ACTH excess with high-dose dex suppression, which is classic for ACTH-secreting pituitary adenoma.
What you’d expect instead:
- Prolactinoma: galactorrhea, amenorrhea, low GnRH → low LH/FSH, high prolactin
- Nonfunctioning adenoma: headaches, bitemporal hemianopsia, hypopituitarism signs
The “Next Best Step” Add-On (Common Follow-Up Question)
Once you suspect Cushing disease:
- Pituitary MRI
- If MRI negative or equivocal (microadenomas can be tiny):
Inferior petrosal sinus sampling (IPSS) is the gold standard to distinguish pituitary vs ectopic ACTH.
IPSS interpretation (high-yield):
- Central (petrosal) ACTH >> peripheral ACTH → pituitary source
- No central gradient → ectopic ACTH
Why Hypokalemic Metabolic Alkalosis Shows Up in Cushing Questions
At high cortisol levels, 11β-HSD2 (the enzyme that normally converts cortisol → cortisone in the kidney) can get overwhelmed. Then cortisol can activate mineralocorticoid receptors → Na⁺ retention, K⁺ loss, H⁺ loss.
So you can see:
- Hypertension
- Hypokalemia
- Metabolic alkalosis
This is especially prominent in ectopic ACTH (often very high cortisol), but it can appear in severe pituitary disease too.
Rapid-Fire High-Yield Takeaways (USMLE-Friendly)
- Cushing syndrome = hypercortisolism from any cause; Cushing disease = pituitary ACTH adenoma.
- First confirm Cushing with urinary free cortisol / late-night salivary cortisol / low-dose dex.
- Then ACTH:
- High ACTH → ACTH-dependent (pituitary vs ectopic)
- Low ACTH → adrenal or exogenous steroids
- High-dose dex suppression:
- Suppresses → pituitary (Cushing disease)
- No suppression → ectopic ACTH or adrenal cause
- Imaging clue:
- ACTH-dependent → bilateral adrenal hyperplasia
- ACTH-independent adrenal tumor → unilateral mass, contralateral atrophy
- Exogenous steroids → bilateral adrenal atrophy
Mini Self-Check (One-Liner Practice)
- Elevated cortisol + low ACTH + unilateral adrenal mass → adrenal adenoma/carcinoma
- Elevated cortisol + high ACTH + no suppression with high-dose dex → ectopic ACTH
- Elevated cortisol + high ACTH + suppression with high-dose dex → Cushing disease