Cushing questions are basically the USMLE’s favorite endocrine “detective story”: the patient has too much cortisol, and your job is to figure out where the signal is coming from (pituitary vs adrenal vs ectopic) and how to prove it with a clean, stepwise workup. Once you nail the logic, these questions become easy points.
Big Picture: Cushing Syndrome vs Cushing Disease
Definitions (know this cold)
- Cushing syndrome = hypercortisolism from any cause (exogenous steroids, pituitary ACTH, adrenal tumor, ectopic ACTH, etc.).
- Cushing disease = pituitary ACTH-secreting adenoma causing hypercortisolism.
- This is a subset of Cushing syndrome.
Step mindset:
- Syndrome = “too much cortisol.”
- Disease = “pituitary ACTH tumor did it.”
HPA Axis Refresher (Why the labs look the way they do)
Normal physiology:
- Hypothalamus: CRH
- Pituitary: ACTH
- Adrenal zona fasciculata: cortisol
- Cortisol provides negative feedback to hypothalamus + pituitary.
When cortisol is high, ask:
- Is ACTH high or low?
- If ACTH is high, is it pituitary (suppressible) or ectopic (not suppressible)?
Pathophysiology by Etiology
1) Exogenous glucocorticoids (most common cause of Cushing syndrome overall)
- Example: chronic prednisone therapy for autoimmune disease, transplant, COPD, etc.
- Mechanism: high steroids → low CRH, low ACTH → bilateral adrenal atrophy
- Key clue: features of Cushing + history of steroid use
2) Cushing disease (pituitary ACTH adenoma)
- ACTH ↑ → bilateral adrenal hyperplasia → cortisol ↑
- Patients may also have mass effect symptoms (headache, visual field defects), but not always.
3) Ectopic ACTH production (classically small cell lung carcinoma)
- ACTH ↑↑ (often very high) → cortisol ↑
- Tends to be more severe and rapid onset
- Not suppressible with high-dose dexamethasone
4) Adrenal cortisol-secreting tumor (adenoma or carcinoma)
- Cortisol ↑ → ACTH ↓ (feedback)
- Often unilateral adrenal mass; contralateral adrenal may atrophy
Clinical Presentation (What cortisol excess does)
Core features (high yield)
- Central obesity, moon facies, buffalo hump
- Thin skin, easy bruising, purple striae
- Proximal muscle weakness (protein catabolism)
- Hyperglycemia / insulin resistance (gluconeogenesis)
- Hypertension
- Cortisol can activate mineralocorticoid receptors at high levels (esp if 11β-HSD2 is overwhelmed)
- Osteoporosis
- Mood changes (depression, irritability, psychosis)
- Infections (immunosuppression), poor wound healing
Androgen-related findings (depends on ACTH)
- If ACTH-dependent (pituitary or ectopic): increased adrenal androgens → hirsutism, acne, menstrual irregularities (more prominent in women)
Pediatric clue
- Weight gain + growth retardation is a classic pediatric hypercortisolism clue.
Diagnosis: The Stepwise USMLE Approach
Step 0: Rule out “fake Cushing” / physiologic hypercortisolism
- Depression, alcoholism (“pseudo-Cushing”), severe obesity can elevate cortisol.
- On exams, they may still test the standard algorithm—just be aware.
Step 1: Screen for hypercortisolism (pick one)
You typically start with one of these:
| Screening test | What you’re looking for | High-yield note |
|---|---|---|
| Overnight low-dose dexamethasone suppression test | Normal: dex suppresses cortisol | In Cushing syndrome, cortisol fails to suppress |
| Late-night salivary cortisol | Loss of diurnal cortisol nadir | Convenient and common |
| 24-hour urinary free cortisol | Total cortisol production | Elevated in Cushing |
Interpretation anchor:
If cortisol is high and does not suppress with low-dose dexamethasone, you have Cushing syndrome (now find the source).
Step 2: Measure ACTH (localize the source)
ACTH low vs high tells you the category
- ACTH low → ACTH-independent → adrenal tumor or exogenous steroids (though exogenous often has low endogenous cortisol too; history is key)
- ACTH high → ACTH-dependent → pituitary (Cushing disease) vs ectopic ACTH
Step 3: Differentiate pituitary ACTH vs ectopic ACTH
High-dose dexamethasone suppression test (classic Step 1 favorite)
- Pituitary adenoma: partially retains feedback → cortisol decreases with high-dose dex
- Ectopic ACTH: no feedback regulation → no suppression
Memory hook (without being cute):
Pituitary is “somewhat obedient,” ectopic is “totally rebellious.”
CRH stimulation test (often tested)
- Pituitary ACTH adenoma: ACTH increases after CRH
- Ectopic ACTH: no significant response
Imaging (after biochemical localization)
- Pituitary MRI for suspected Cushing disease
- CT chest/abdomen for ectopic sources (e.g., small cell lung cancer)
- Adrenal CT/MRI for ACTH-independent causes
Putting It All Together: HY Lab Patterns Table
| Condition | ACTH | Cortisol | Dexamethasone (low dose) | Dexamethasone (high dose) | Adrenal glands |
|---|---|---|---|---|---|
| Cushing disease (pituitary ACTH adenoma) | ↑ | ↑ | No suppression | Suppresses (↓ cortisol) | Bilateral hyperplasia |
| Ectopic ACTH (e.g., small cell lung cancer) | ↑↑ | ↑ | No suppression | No suppression | Bilateral hyperplasia |
| Adrenal tumor (adenoma/carcinoma) | ↓ | ↑ | No suppression | No suppression | Unilateral tumor, contralateral atrophy |
| Exogenous glucocorticoids | ↓ | (Endogenous) ↓ | N/A | N/A | Bilateral atrophy |
Exam trap: Exogenous steroids can produce Cushingoid appearance but endogenous cortisol may be low because the assay may not measure synthetic steroids well; rely on history + ACTH suppression + adrenal atrophy.
Treatment (Etiology-specific and testable)
General principle
- Treat the underlying cause and manage complications (HTN, diabetes, osteoporosis, infections).
Cushing disease (pituitary adenoma)
- Transsphenoidal resection is first-line.
- If not surgical candidate or persistent disease:
- Pituitary radiation
- Medical therapy to reduce cortisol (e.g., steroidogenesis inhibitors like ketoconazole, metyrapone; or glucocorticoid receptor antagonist like mifepristone—less commonly tested but fair game)
Ectopic ACTH
- Treat the tumor (e.g., small cell lung cancer).
- Medical cortisol control may be needed as bridge therapy.
Adrenal tumor
- Adrenalectomy (and evaluate for carcinoma if suspicious).
Exogenous steroids
- Gradual taper (avoid adrenal crisis due to HPA axis suppression).
High-Yield Associations & Classic Vignettes
1) Small cell lung carcinoma → ectopic ACTH
- Smoking history, weight loss, lung mass
- Profound hypercortisolism, often hypokalemic metabolic alkalosis can show up due to mineralocorticoid effects
2) Pituitary adenoma → Cushing disease
- ACTH-dependent hypercortisolism
- May see other pituitary issues or mass effect, but many questions rely on high-dose dex suppression and CRH response
3) Adrenal adenoma/carcinoma
- ACTH low
- Imaging shows adrenal mass
- Consider co-secretion patterns in broader adrenal pathology questions (though primary hyperaldosteronism is a separate topic)
4) Steroid therapy complication
- Cushingoid appearance + infections + osteoporosis
- Suppressed ACTH and adrenal atrophy; taper is key
“First Aid” Cross-References (by concept)
(Page numbers vary by edition—use these as index targets.)
- Adrenal cortex hormones & zones: zona fasciculata → cortisol (Endocrine section)
- Cushing syndrome vs Cushing disease: endocrine pathology tables + pituitary/adrenal disorders
- Dexamethasone suppression testing patterns: endocrine diagnostics tables
- Ectopic ACTH (small cell lung carcinoma): integrated pathology (pulmonary malignancies + paraneoplastic syndromes)
- Exogenous steroid effects: immunosuppression + adrenal atrophy; pharmacology of glucocorticoids
Rapid-Fire USMLE Checklist (what to recall under time pressure)
- Cushing syndrome = hypercortisolism from any cause
- Cushing disease = pituitary ACTH adenoma
- Low-dose dex: screens (Cushing → no suppression)
- ACTH level localizes:
- ACTH low → adrenal/exogenous
- ACTH high → pituitary vs ectopic
- High-dose dex:
- Pituitary → suppresses
- Ectopic → no suppression
- Ectopic ACTH: think small cell lung carcinoma
- Exogenous steroids: low ACTH, adrenal atrophy, taper slowly