55.2 Adrenal Disorders: Addison's Disease & Cushing's Syndrome

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55 Chapter 55: Chronic Endocrine Disorders

55.1 Primary & Subclinical Hypothyroidism, Hyperthyroidism & Nodules 55.2 Adrenal Disorders: Addison's Disease & Cushing's Syndrome 55.3 Hypercalcemia, Hyperparathyroidism & Pituitary Disorders

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55.2 Adrenal Disorders: Addison's Disease & Cushing's Syndrome

Key Takeaways Explain This Topic AI

Primary adrenal insufficiency (Addison disease) results from destructive lesions of the entire adrenal cortex (80-90% autoimmune adrenalitis with 21-hydroxylase autoantibodies), causing combined deficiencies of cortisol and aldosterone that produce hyponatremia, hyperkalemia, non-anion gap metabolic acidosis, orthostatic hypotension, salt craving, and pathognomonic mucocutaneous hyperpigmentation driven by uninhibited ACTH and pro-opiomelanocortin cleavage.

Diagnostic confirmation of adrenal insufficiency requires an 8 AM morning cortisol (<3-5 mcg/dL highly suspicious) followed by the high-dose (250 mcg) cosyntropin (synthetic ACTH) stimulation test; failure of peak cortisol to rise >=18 mcg/dL at 30 or 60 minutes confirms adrenal failure, and an elevated baseline plasma ACTH (>2x ULN) distinguishes primary adrenal destruction from secondary central hypopituitarism (where aldosterone is preserved by the intact RAAS).

Acute adrenal crisis is a life-threatening, refractory vasodilatory shock presenting with fever, severe abdominal pain, and vomiting; management mandates immediate IV hydrocortisone 100 mg bolus every 8 hours paired with rapid 0.9% normal saline with 5% dextrose resuscitation without delaying for diagnostic testing (if cosyntropin testing is simultaneously executed, administer dexamethasone 4 mg IV, which does not cross-react with cortisol immunoassays).

Chronic maintenance for Addison disease requires physiological glucocorticoid replacement (hydrocortisone 15-25 mg/day in divided doses, taking two-thirds in the morning) plus mineralocorticoid replacement (fludrocortisone 0.05-0.2 mg daily); patients must be trained in sick-day rules (doubling or tripling oral doses during febrile illnesses), carry an injectable emergency hydrocortisone 100 mg IM kit, and wear medical alert identification.

Cushing syndrome (hypercortisolism) requires confirmation by at least two positive first-line screening tests (24-hour urinary free cortisol, late-night salivary cortisol, or 1-mg overnight dexamethasone suppression test with 8 AM cortisol >1.8 mcg/dL); plasma ACTH subsequent testing stratifies ACTH-dependent causes (pituitary Cushing disease vs ectopic ACTH; ACTH >20 pg/mL) from ACTH-independent causes (adrenal adenoma or carcinoma; ACTH <5 pg/mL).

Last updated: September 2026

Primary vs. Secondary Adrenal Insufficiency: Pathophysiology & Clinical Manifestations

Adrenal insufficiency encompasses a spectrum of endocrine disorders characterized by inadequate adrenocortical steroid hormone production. Proper clinical management requires distinguishing primary adrenal insufficiency (intrinsic adrenal gland destruction) from secondary and tertiary adrenal insufficiency (central pituitary ACTH or hypothalamic CRH deficiency).

Functional Adrenal Zonation & Endocrine Physiology

The adrenal cortex is structurally organized into three distinct concentric histological zones, each governing specific steroidogenic pathways: HISTOLOGICAL ZONATION OF THE ADRENAL GLAND

Zone Primary Hormone Produced Primary Regulatory Pathway
═════════════════════════════════════════════════════════════════════════════════════════════
Zona Glomerulosa Mineralocorticoids (Aldosterone) Renin-Angiotensin-Aldosterone System
(Outer 15%) (RAAS) & Serum Potassium [ACTH-Independent!]

Zona Fasciculata Glucocorticoids (Cortisol) Hypothalamic CRH ──► Pituitary ACTH
(Middle 75%) [Subject to negative feedback]

Zona Reticularis Adrenal Androgens (DHEA, DHEA-S) Pituitary ACTH
(Inner 10%)

Adrenal Medulla Catecholamines (Epinephrine, NE) Sympathetic preganglionic chromaffin cells
═════════════════════════════════════════════════════════════════════════════════════════════
Primary Adrenal Insufficiency (Addison Disease): Etiologies

Addison disease is characterized by destruction of >=90% of both adrenal cortices , compromising all three cortical zones (loss of glucocorticoids, mineralocorticoids, and adrenal androgens):

Autoimmune Adrenalitis : Accounts for 80% to 90% of cases in developed nations. Characterized by circulating autoantibodies directed against the 21-hydroxylase enzyme (CYP21A2), leading to cytotoxic T-cell-mediated lymphocytic adrenal atrophy. May occur as an isolated entity or as part of Autoimmune Polyglandular Syndromes:

APS-1 (APECED) : Autosomal recessive mutation in the AIRE gene on chromosome 21. Triad of chronic mucocutaneous candidiasis, hypoparathyroidism, and Addison disease .

APS-2 (Schmidt Syndrome) : Polygenic inheritance associated with HLA-DR3/DR4. Cluster of Addison disease, autoimmune thyroid disease (Hashimoto or Graves), and Type 1 Diabetes Mellitus .

Infectious Adrenalitis :

Tuberculosis : Historically the leading cause and remains the most common etiology worldwide in developing nations. Chronic granulomatous infection causes enlarged, calcified adrenal glands visible on abdominal CT.

Fungal Infections : Disseminated histoplasmosis, coccidioidomycosis, and blastomycosis in immunocompromised hosts.

Opportunistic Infections : Cytomegalovirus (CMV), Mycobacterium avium complex (MAC), and cryptococcosis in advanced HIV/AIDS.

Bilateral Adrenal Hemorrhage & Infarction :

Waterhouse-Friderichsen Syndrome : Catastrophic fulminant meningococcemia ( Neisseria meningitidis ) or pneumococcal sepsis causing severe endotoxemia, disseminated intravascular coagulation (DIC), widespread purpura, and acute bilateral hemorrhagic adrenal destruction.

Antiphospholipid antibody syndrome (APLS) and Heparin-Induced Thrombocytopenia (HIT).

Infiltrative & Metastatic Disease : Metastatic spread to the adrenals from primary lung cancer, breast carcinoma, malignant melanoma, or renal cell carcinoma; bilateral lymphoma; amyloidosis; hemochromatosis.

Pharmacologic Inhibition of Steroidogenesis :

Etomidate : A single induction dose for endotracheal intubation transiently inhibits 11-beta-hydroxylase, precipitating acute adrenal insufficiency in critically ill septic patients.

Ketoconazole : Inhibits CYP17 and CYP11A1 at high antifungal doses.

Immune Checkpoint Inhibitors : Anti-CTLA-4 (ipilimumab) causes autoimmune hypophysitis; anti-PD-1 (pembrolizumab, nivolumab) causes primary adrenalitis.

Secondary & Tertiary Adrenal Insufficiency: The RAAS Preserved

Secondary Insufficiency : Pituitary disease impairs ACTH secretion (pituitary macroadenomas, craniopharyngiomas, pituitary surgery/radiation, Sheehan syndrome, or pituitary apoplexy).

Tertiary Insufficiency : Hypothalamic suppression of CRH, overwhelmingly caused by the abrupt cessation or rapid taper of chronic exogenous glucocorticoid therapy (suppressing the hypothalamic-pituitary-adrenal [HPA] axis for weeks to months).

THE FUNDAMENTAL PATHOPHYSIOLOGIC DISTINCTION :

In secondary and tertiary adrenal insufficiency, the adrenal glands are structurally intact, and the RAAS functions normally because aldosterone secretion is regulated by angiotensin II and hyperkalemia, not ACTH.

Therefore, patients with central (secondary/tertiary) insufficiency suffer from isolated cortisol and androgen deficiency . They DO NOT develop aldosterone deficiency ; hyperkalemia, severe hypovolemic shock, and severe salt craving are absent.

Furthermore, because pituitary ACTH secretion is low or absent, hyperpigmentation never develops .

Clinical Presentation & Diagnostic Protocol: The Cosyntropin Stimulation Test
Clinical Manifestations of Chronic Primary Adrenal Insufficiency

The clinical onset of Addison disease is notoriously insidious, often progressing unrecognized over months or years until precipitated into an acute crisis by an intercurrent illness:

Glucocorticoid Deficiency : Severe chronic fatigue, generalized muscular asthenia, anorexia, progressive involuntary weight loss (present in 100% of patients), gastrointestinal distress (nausea, vomiting, vague abdominal pain, intermittent diarrhea), hypoglycemia (loss of cortisol-mediated gluconeogenesis and hepatic glycogenolysis), and neuropsychiatric disturbances (apathy, depression, irritability).

Mineralocorticoid Deficiency : Severe orthostatic hypotension, postural lightheadedness, syncope, and intense salt craving (present in 20% to 30% of patients; craving salty foods, pickles, soy sauce due to persistent renal sodium wasting).

Androgen Deficiency : In women, the adrenal cortex is the primary source of circulating androgens; deficiency manifests as loss of pubic and axillary hair, dry pruritic skin, and loss of libido. (In men, testicular Leydig cells produce adequate testosterone, sparing them from androgen deficiency symptoms).

Mucocutaneous Hyperpigmentation (The Cardinal Clinical Sign) :

Pathophysiology: Loss of cortisol negative feedback prompts massive compensatory transcription of the pro-opiomelanocortin (POMC) gene in anterior pituitary corticotrophs. POMC is enzymatically cleaved into ACTH and melanocyte-stimulating hormone (alpha-MSH) . Both hormones bind to melanocortin-1 receptors (MC1R) on dermal and mucosal melanocytes, driving melanin synthesis.

Distribution: Prominent in areas exposed to sunlight (face, neck, dorsum of hands) and areas subject to mechanical friction or pressure (elbows, knees, knuckles, belt lines, palmar creases), the vermilion border of the lips, the buccal mucosa (bluish-black patches), areolae, perineum, and recent surgical scars.

Laboratory Abnormalities in Addison Disease

Electrolyte Hallmarks :

Hyponatremia (85% to 90%) : Dual mechanism: 1) Aldosterone deficiency produces renal sodium wasting in the cortical collecting tubule; 2) Cortisol deficiency removes tonic inhibition of hypothalamic antidiuretic hormone (ADH) secretion, causing non-osmotic ADH hypersecretion and renal free water retention.

Hyperkalemia (60% to 70%) : Aldosterone deficiency prevents principal cell sodium reabsorption, impairing the luminal electrical gradient required for potassium excretion in the urine.

Non-Anion Gap (Normal Anion Gap) Metabolic Acidosis : Impaired aldosterone-mediated proton ( H + H^+ H + ) secretion by intercalated cells produces a distal hyperchloremic metabolic acidosis.

Complete Blood Count & Metabolic Labs : Moderate normocytic normochromic anemia, relative lymphocytosis, and eosinophilia (cortisol normally promotes apoptosis of circulating eosinophils; unexplained eosinophilia in a hypotensive, fatigued patient is a classic board clue); elevated blood urea nitrogen (BUN) and creatinine (prerenal azotemia from hypovolemia); fasting hypoglycemia.

Stepwise Diagnostic Protocol for Adrenal Insufficiency DIAGNOSTIC ALGORITHM FOR ADRENAL INSUFFICIENCY

Suspected Adrenal Insufficiency (Fatigue, Weight Loss,
Hypotension, Hyponatremia, Hyperpigmentation)
│
▼
Measure 8:00 AM Serum Cortisol & Plasma ACTH
│
┌──────────────────────────┼──────────────────────────┐
▼ ▼ ▼
< 3.0 mcg/dL 3.0 to 14.9 mcg/dL >= 15.0 to 18.0 mcg/dL
Highly Suggestive of Indeterminate Baseline Adrenal Insufficiency
Adrenal Insufficiency Cortisol Level Virtually Excluded
│ │
└──────────────────────────┴──────────────────────────┐
│
▼
High-Dose Cosyntropin Stimulation Test
(250 mcg Synthetic ACTH IV or IM)
Measure Cortisol at 0, 30, and 60 Minutes
│
┌──────────────────────┴──────────────────────┐
▼ ▼
Peak Cortisol >= 18 mcg/dL Peak Cortisol < 18 mcg/dL
Normal Adrenal Response CONFIRMED ADRENAL INSUFFICIENCY
(Adrenal Failure Excluded) │
▼
Evaluate Baseline Plasma ACTH
│
┌────────────────────────────────┴────────────────────────────────┐
▼ ▼
Baseline Plasma ACTH HIGH Baseline Plasma ACTH LOW
(> 2x ULN; > 50-100 pg/mL) or Inappropriately Normal
│ │
▼ ▼
PRIMARY ADRENAL INSUFFICIENCY SECONDARY / TERTIARY AI
(Addison Disease) (Central Pituitary/Hypothalamic)
Check 21-Hydroxylase Antibodies • Evaluate Pituitary MRI
If negative: Adrenal CT Scan • Evaluate other pituitary axes

Step 1: Baseline 8:00 AM Serum Cortisol :

Must be measured at the circadian zenith (between 7:00 AM and 9:00 AM).

A value < 3.0 mcg/dL is virtually diagnostic of adrenal insufficiency.

A value >= 15 to 18 mcg/dL reliably excludes adrenal insufficiency in non-critically ill outpatients.

Intermediate values (3.0 to 14.9 mcg/dL) are indeterminate and mandate provocative testing.

Step 2: The High-Dose Cosyntropin (Synthetic A C T H 1 − 24 ACTH_{1-24} A CT H 1 − 24 ) Stimulation Test :

The definitive gold-standard provocative test. Administer 250 mcg of cosyntropin IV or IM.

Measure serum cortisol concentrations at baseline (0 min), 30 minutes, and 60 minutes post-injection.

Normal Response : Serum cortisol rises to a peak concentration >= 18.0 to 20.0 mcg/dL at 30 or 60 minutes.

Abnormal Response (Diagnostic of Adrenal Insufficiency) : Peak cortisol fails to reach 18.0 mcg/dL, reflecting atrophic or destroyed adrenal cortices incapable of steroid synthesis.

Step 3: Differentiating Primary from Secondary Insufficiency :

Guided by the baseline plasma ACTH concentration drawn prior to cosyntropin administration:

Primary Adrenal Insufficiency : Plasma ACTH is markedly elevated (typically >100 to 200 pg/mL , well above the reference range of 10-60 pg/mL) due to complete absence of cortisol negative feedback.

Secondary/Tertiary Insufficiency : Plasma ACTH is low or inappropriately normal (<10-20 pg/mL) despite a low cortisol level.

Acute Adrenal Crisis: Emergency Recognition & Immediate Resuscitation

Acute adrenal crisis (Addisonian crisis) is an immediately life-threatening endocrine emergency characterized by severe acute glucocorticoid and mineralocorticoid deficiency, culminating in hypovolemic, vasodilatory shock refractory to fluid administration and vasopressors .

Clinical Triggers & Presentation

Precipitating Triggers : Acute physiological stress superimposed on chronic undiagnosed Addison disease or missed maintenance doses in a known patient. Common precipitants include systemic infections (sepsis, pneumonia, gastroenteritis), major surgical procedures, trauma, severe burns, prolonged vomiting/diarrhea, myocardial infarction, or hypothermia.

Clinical Signs : Severe intractable hypotension and circulatory collapse; high fever (103°F-105°F [39.5°C-40.5°C]); acute, severe abdominal pain that closely mimics an acute surgical abdomen (with guarding, rigidity, and rebound tenderness); continuous nausea and projectile vomiting; flank pain; altered mental status ranging from confusion and delirium to stupor and coma.

Emergency Management Protocol: The Three Pillars of Crisis Care ACUTE ADRENAL CRISIS EMERGENCY RESUSCITATION

Intervention Specific Protocol & Clinical Instructions
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Intravenous • Establish 2 large-bore (16-18 gauge) peripheral IV lines immediately
Hydration • Infuse 0.9% Normal Saline with 5% Dextrose (D5NS) at 1,000 mL/hr
for the first 1-2 hours, then titrate to hemodynamics (typically
2-3 liters within the first 4-6 hours)
Normal saline restores intravascular volume; 5% dextrose treats hypoglycemia

Parenteral • Hydrocortisone 100 mg IV STAT bolus, followed by 50 to 100 mg IV
Glucocorticoids every 6 to 8 hours (or 200 mg/24 hours by continuous IV infusion)
At doses >=100 mg/day, hydrocortisone provides 100% saturation of

mineralocorticoid receptors; ADDING FLUDROCORTISONE IS UNNECESSARY!

Testing Rule: • NEVER delay life-saving glucocorticoid administration to perform

Dexamethasone diagnostic cosyntropin testing!

Alternative • If the diagnosis is unconfirmed and cosyntropin testing can be performed

STAT, administer DEXAMETHASONE 4 mg IV bolus instead of hydrocortisone.

• Dexamethasone does not cross-react in serum cortisol radioimmunoassays

or chemiluminescent assays, allowing accurate cortisol measurement.

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Chronic Maintenance Pharmacotherapy & Adrenal Patient Education

Once an adrenal crisis is resolved or chronic primary adrenal insufficiency is diagnosed, patients require lifelong dual-hormone replacement therapy designed to replicate normal circadian physiological fluctuations.

Chronic Replacement Regimen

Glucocorticoid Replacement :

Hydrocortisone (Oral) : The physiological drug of choice. Daily maintenance dose is 15 to 25 mg total , administered in divided doses to simulate normal diurnal cortisol secretion (which peaks at 8:00 AM and reaches a nadir at midnight):

Two-Thirds / One-Third Regimen : 10 to 15 mg taken immediately upon waking in the morning, and 5 to 10 mg taken in the early afternoon (typically 2:00 PM to 3:00 PM).

Evening doses (after 5:00 PM) are avoided to prevent insomnia and metabolic side effects.

Monitoring Glucocorticoid Replacement : Driven entirely by clinical evaluation (energy level, well-being, appetite, blood pressure, weight maintenance, and absence of Cushingoid features). Serum cortisol and plasma ACTH levels are completely unhelpful for dose titration and should never be used to titrate hydrocortisone.

Mineralocorticoid Replacement :

Fludrocortisone Acetate : Required in all patients with primary adrenal insufficiency (never in secondary insufficiency). Usual maintenance dose is 0.05 to 0.2 mg orally once daily in the morning .

Monitoring Fludrocortisone Replacement : Titrated to maintain seated and standing blood pressure without orthostasis, normalize serum sodium and potassium, and maintain plasma renin activity (PRA) in the upper-normal reference range . Instruct patients to maintain liberal dietary sodium intake (do not restrict salt).

Androgen Replacement (Optional) :

Dehydroepiandrosterone (DHEA) : 25 to 50 mg daily orally. Indicated selectively in women experiencing persistent fatigue, low libido, depressed mood, and reduced muscle strength despite optimized glucocorticoid and mineralocorticoid dosing.

Mandatory Patient Education & "Sick-Day Rules"

Preventing fatal adrenal crisis requires comprehensive patient and family self-management education:

Sick-Day Rule 1 (Mild-to-Moderate Illness) :

For febrile illnesses with temperature >100.4°F (38°C), acute bacterial infections requiring antibiotics, or minor dental procedures: Immediately double (or triple) the oral hydrocortisone dose (e.g., 40 to 60 mg/day in 3 divided doses).

Continue the elevated dose until the fever and acute illness resolve (typically 2 to 3 days), then return directly to baseline maintenance without a prolonged taper.

Sick-Day Rule 2 (Severe Illness or Emesis) :

If the patient develops persistent vomiting, severe diarrhea, major trauma, or cannot retain oral medications : The patient or a trained family member must immediately inject Hydrocortisone 100 mg IM from an emergency self-injection kit (Solu-Cortef Act-O-Vial) and immediately call emergency services (911) or proceed to the nearest emergency department.

Major Surgery / Procedural Stress Dosing :

Minor outpatient procedures under local anesthesia: Double oral dose for 24 hours.

Major surgery requiring general anesthesia: Hydrocortisone 100 mg IV immediately prior to anesthesia induction, followed by 50 mg IV every 8 hours for 24 to 48 hours, rapidly tapering by 50% daily back to oral baseline as oral intake resumes.

Medical Alert Identification :

Every patient with adrenal insufficiency must continuously wear a MedicAlert bracelet or necklace stamped with: "Adrenal Insufficiency: Requires Daily Steroids. In Emergency, Give Hydrocortisone 100 mg IV/IM STAT." Patients must also carry an emergency medical information card in their wallet.

Cushing Syndrome (Hypercortisolism): Etiology, Clinical Features & Diagnostic Algorithm

Cushing syndrome comprises the constellation of clinical and biochemical manifestations resulting from prolonged, inappropriate tissue exposure to supraphysiologic levels of free glucocorticoids.

Etiologic Classification of Hypercortisolism ETIOLOGIC CLASSIFICATION OF CUSHING SYNDROME

Classification Specific Etiology Relative Frequency Key Diagnostic Markers
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Exogenous Iatrogenic administration of > 80% of ALL cases Suppressed morning ACTH;
(Most Common) oral, inhaled, topical, or overall suppressed adrenal cortisol;
intra-articular steroids bilateral adrenal atrophy

ACTH-Dependent Cushing Disease ~ 70% of endogenous Elevated/normal ACTH;
(80-85% of (Pituitary ACTH-secreting cases (8:1 female suppresses on high-dose DST;
Endogenous) corticotroph adenoma) predominance) positive IPSS gradient (>3:1)

Ectopic ACTH Secretion ~ 10-15% of Markedly elevated ACTH;
(Small cell lung carcinoma; endogenous cases severe hypokalemia / alkalosis;
bronchial carcinoid; panNET) (male predominance) fails high-dose DST suppression

ACTH-Independent Unilateral Adrenal Adenoma ~ 10% of endogenous Suppressed plasma ACTH (<5 pg/mL);
(15-20% of (Autonomous cortisol secretion)cases unilateral adrenal mass on CT
Endogenous)
Adrenocortical Carcinoma ~ 5% of endogenous Suppressed ACTH; large mass (>4-6 cm);
(Highly malignant; aggressive) cases co-secretes androgens (DHEA-S/virilization)
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Clinical Manifestations: Discriminating Features

Because many symptoms of Cushing syndrome overlap with common conditions (obesity, hypertension, metabolic syndrome), clinicians must search for the specific discriminating clinical features that possess high diagnostic specificity for hypercortisolism:

High Specificity Discriminating Physical Features :

Violaceous Wide Striae : Deep reddish-purple cutaneous stretch marks > 1 cm in width located predominantly on the abdomen, flanks, breasts, and thighs (distinguished from pale, narrow silver striae seen in simple obesity).

Cutaneous Atrophy & Easy Bruising : Paper-thin skin with extensive ecchymoses following minor trauma, skin fragility, and impaired wound healing (resulting from cortisol-mediated collagen lysis and subcutaneous connective tissue destruction).

Proximal Muscle Myopathy : Painless, progressive weakness affecting proximal limb girdle musculature (difficulty rising from a chair without using hands, difficulty climbing stairs, or lifting arms above head), caused by profound catabolic muscle protein wasting. Deep tendon reflexes and sensation are entirely preserved.

Facial Plethora : Reddish-purple flushed engorgement of the cheeks, distinct from simple rosacea.

Additional Classic Features :

Centripetal Adiposity : Progressive central obesity with disproportionately thin extremities; rounded "moon facies"; prominent dorsocervical fat pad ("buffalo hump"); and supraclavicular fat pad fullness.

Cardiovascular & Metabolic : Secondary hypertension (80% of patients), impaired fasting glucose or overt secondary Type 2 Diabetes Mellitus, hyperlipidemia.

Skeletal Fragility : Severe osteopenia, osteoporosis, and spontaneous low-trauma vertebral compression fractures or rib fractures in young individuals.

Reproductive & Dermatologic : Menstrual irregularities (amenorrhea or oligomenorrhea), hirsutism, cystic acne, androgenic alopecia (driven by adrenal androgen co-secretion in women).

Neuropsychiatric : Emotional lability, major depression, panic disorder, severe insomnia, cognitive decline, and frank steroid-induced psychosis.

Stepwise Diagnostic & Localization Algorithm for Hypercortisolism

Investigating suspected Cushing syndrome requires a systematic 4-step framework: CUSHING SYNDROME DIAGNOSTIC & LOCALIZATION FRAMEWORK

STEP 1: Rule Out Exogenous Corticosteroids
(Rigorously review oral, inhaled, injected, topical meds)
│
▼
STEP 2: Establish Hypercortisolism (Screening)

Must obtain AT LEAST TWO ABNORMAL First-Line Screening Tests:

1. 24-Hour Urinary Free Cortisol (UFC > 3x ULN)
2. Late-Night Salivary Cortisol (>= 2 separate nights; elevated)
3. Overnight 1-mg Dexamethasone Suppression Test (8 AM Cortisol > 1.8 mcg/dL)
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▼
STEP 3: Determine ACTH Dependence (Plasma ACTH)
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┌──────────────────────────┴──────────────────────────┐
▼ ▼
Plasma ACTH < 5 pg/mL Plasma ACTH > 20 pg/mL
ACTH-INDEPENDENT (Adrenal Source) ACTH-DEPENDENT (Pituitary vs Ectopic)
│ │
▼ ▼
High-Resolution Adrenal CT/MRI STEP 4: Localize ACTH Source
│ │
┌─────┴─────┐ ┌─────────────┴─────────────┐
▼ ▼ ▼ ▼
Unilateral Adrenal Carcinoma High-Dose (8-mg) DST Pituitary MRI with Gadolinium
Adenoma (Large >4-6 cm, (Cortisol suppresses >50% (Adenoma identified >=6 mm)
(Laparoscopic irregular margins) in Pituitary Cushing; │
Surgery) fails to suppress in Ectopic) ▼

│ If MRI negative or <6 mm:

└───────────┬───────────┘
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▼
Bilateral Inferior Petrosal Sinus
Sampling (IPSS) with IV CRH Stimulation
Petrosal-to-Peripheral ACTH Ratio > 3:1

CONFIRMS PITUITARY CUSHING DISEASE!

Ratio < 2:1 to 3:1 = ECTOPIC ACTH TUMOR
(Obtain Neck/Chest/Abdomen CT/PET)

Step 1: Exclude Exogenous Glucocorticoids :

Meticulously review prescription, over-the-counter, and herbal supplements (including joint injections, skin-lightening creams, and asthma inhalers).

Step 2: Confirm Pathologic Hypercortisolism (The 3 First-Line Tests) :

Guidelines mandate at least two concordant abnormal screening tests before hypercortisolism is diagnosed:

24-Hour Urinary Free Cortisol (UFC) : Quantifies integrated 24-hour unbound, biologically active cortisol filtration. Diagnostic threshold: UFC > 3 times the upper limit of normal (>150-300 mcg/24h). Requires concomitant urine creatinine measurement to confirm collection adequacy.

Late-Night Salivary Cortisol (LNSC) : Evaluates loss of the normal circadian nadir. Cortisol secretion physiologically drops to near-undetectable levels between 11:00 PM and midnight. Elevated salivary cortisol on >=2 separate nights establishes loss of circadian rhythm.

Overnight 1-mg Dexamethasone Suppression Test (1-mg DST) : Administer 1 mg of oral dexamethasone at 11:00 PM; measure fasting serum cortisol at 8:00 AM the next morning. In healthy individuals, dexamethasone suppresses pituitary ACTH, driving 8:00 AM cortisol <1.8 mcg/dL (50 nmol/L). An 8:00 AM cortisol > 1.8 mcg/dL represents abnormal non-suppression.

Confounding Caveat : Estrogen-containing oral contraceptives increase thyroxine-binding globulin and corticosteroid-binding globulin (CBG), producing false-positive total serum cortisol elevations on the 1-mg DST. Stop oral contraceptives 6 weeks prior, or use UFC/salivary cortisol instead.

Step 3: Measure Baseline Plasma ACTH :

Plasma ACTH < 5 pg/mL : ACTH-Independent Cushing Syndrome. Autonomous adrenal cortisol secretion completely suppresses pituitary corticotrophs. Next step: Dedicated Thin-Cut Adrenal CT or MRI .

Plasma ACTH > 20 pg/mL : ACTH-Dependent Cushing Syndrome. Autonomous ACTH drives bilateral adrenal hyperplasia. Next step: Proceed to Step 4 to differentiate pituitary from ectopic sources.

Plasma ACTH 5 to 20 pg/mL : Indeterminate gray zone; perform a CRH stimulation test.

Step 4: Differentiating Pituitary Cushing Disease from Ectopic ACTH :

High-Dose Dexamethasone Suppression Test (8-mg DST) : Pituitary corticotroph adenomas retain partial receptor sensitivity; high-dose dexamethasone suppresses morning cortisol by > 50% from baseline . In contrast, ectopic neuroendocrine tumors (e.g., small cell lung cancer) are fully autonomous and fail to suppress (<50% suppression).

Pituitary MRI with Gadolinium : Identifies adenomas in ~60% of Cushing disease cases (frequently microadenomas <10 mm).

Bilateral Inferior Petrosal Sinus Sampling (IPSS) : The definitive diagnostic gold standard when pituitary MRI is negative, equivocal, or demonstrates a microadenoma <6 mm. Catheters are guided into both inferior petrosal sinuses (which drain the anterior pituitary) and a peripheral vein. ACTH is measured before and after IV CRH administration:

Central-to-Peripheral ACTH Ratio > 2:1 at baseline OR > 3:1 after CRH stimulation : Definitive confirmation of Cushing Disease (pituitary source) and lateralizes the microadenoma to the left or right pituitary lobe.

Central-to-Peripheral ACTH Ratio < 2:1 to 3:1 : Confirms Ectopic ACTH Secretion . Next step: High-resolution CT of the chest, abdomen, and pelvis, and Gallium-68 DOTATATE PET scan to locate the occult neuroendocrine tumor.