🩺 Dexamethasone Suppression Test Simulator

The Hypothalamic-Pituitary-Adrenal Feedback Loop

Cortisol normally suppresses its own release via negative feedback.

• Hypothalamus: CRH source (paraventricular nucleus)
• Anterior pituitary: ACTH source (corticotroph cells)
• Adrenal cortex: Cortisol source (zona fasciculata)
• 5–23 µg/dL: Normal AM cortisol (diurnal peak at 8am)

Negative feedback keeps cortisol in a tight range

Hypothalamus releases CRH, pituitary releases ACTH, adrenal makes cortisol. Cortisol then feeds back to suppress CRH and ACTH.

Exogenous glucocorticoid mimics cortisol and shuts the axis down.

Low-Dose Dexamethasone — The Screening Test

1 mg dexamethasone at 11pm; cortisol drawn at 8am the next day.

• 1 mg PO: Standard dose (overnight, 11pm)
• <1.8 µg/dL: Normal suppression (8am cortisol cutoff)
• ~95%: Sensitivity (for Cushing's syndrome)
• Obesity, stress: False positives (depression, OCPs)

Failure to suppress signals autonomous cortisol production

Dexamethasone does not bind the assay, so a positive test is real. Cushing's patients keep making cortisol despite the pituitary being suppressed.

A single failed low-dose test does not localize the cause yet.

High-Dose Dexamethasone — Localizing the Source

8 mg overnight distinguishes pituitary disease from ectopic or adrenal tumors.

• 8 mg PO: Standard dose (overnight, 11pm)
• >50% suppression: Pituitary response (still feedback-sensitive)
• No suppression: Ectopic/adrenal (autonomous source)
• Confirmed Cushing's: Classic use (only, after low-dose fails)

Pituitary adenomas retain partial feedback sensitivity

A pituitary corticotroph tumor still has some glucocorticoid receptors. Ectopic tumors and adrenal tumors are almost fully autonomous.

Modern workup often pairs this with ACTH and imaging.

Comparing Pre- and Post-Dexamethasone Cortisol

Plotting all three scenarios reveals the diagnostic suppression pattern.

• ~90% drop: Normal, low-dose (strong suppression)
• ~25% drop: Pituitary, low-dose (inadequate suppression)
• ~70% drop: Pituitary, high-dose (partial suppression)
• ~15% drop: Ectopic/adrenal, high-dose (resistant to suppression)

Suppression magnitude is the key diagnostic signal

Bar height drop from baseline to post-dex tells the whole story. Bigger drop means feedback is intact; small drop means autonomy.

Use both slider controls to explore every combination.

Interpreting the Full Suppression Pattern

Low- and high-dose results together confirm and localize Cushing's syndrome.

• Suppresses at low dose: Normal pattern (axis intact)
• Fails low, passes high: Pituitary pattern (Cushing disease)
• Fails both doses: Ectopic/adrenal pattern (autonomous source)
• ACTH + imaging: Next step (MRI pituitary / CT chest-abd)

From screening to source localization

Suppression testing separates normal physiology from true Cushing's. High-dose results then point toward pituitary versus ectopic/adrenal source.

Always confirm with plasma ACTH and cross-sectional imaging.