Hypothalamic-Pituitary-Adrenal (HPA) Axis: Morning Cortisol, ACTH, Synacthen Stimulation, and Dexamethasone Suppression

The Hypothalamic-Pituitary-Adrenal (HPA) axis is the principal neuroendocrine feedback loop mediating systemic adaptation to physiological and psychological stress. The axis operates via a three-tiered hierarchical cascade:

$Hypothalamic Paraventricular Nucleus (CRH / AVP) longrightarrow Anterior Pituitary Corticotrophs (ACTH) longrightarrow Adrenal Zona Fasciculata (Cortisol)$

1. Corticotropin-Releasing Hormone (CRH) & Arginine Vasopressin (AVP): Secreted by the paraventricular nucleus into the hypophyseal portal circulation in response to endogenous circadian clocks, systemic cytokines (IL-1, IL-6, TNF-α), hypoglycemia, and acute stress.
2. Adrenocorticotropic Hormone (ACTH / Corticotropin): Cleaved from the pro-hormone pro-opiomelanocortin (POMC) in anterior pituitary corticotroph cells. ACTH binds to the melanocortin-2 receptor (MC2R) on the adrenal cortex, stimulating steroidogenesis via cyclic AMP and steroidogenic acute regulatory (StAR) protein activation. POMC cleavage also produces α-melanocyte-stimulating hormone (α-MSH), which binds melanocortin-1 receptors (MC1R) in epidermal melanocytes, causing cutaneous hyperpigmentation in states of prolonged, high-level ACTH excess.
3. Cortisol (Hydrocortisone): Glucocorticoid synthesized exclusively in the zona fasciculata of the adrenal cortex from cholesterol. Circulates in plasma primarily bound to Corticosteroid-Binding Globulin (CBG / Transcortin, ~85–90%) and albumin (~5–10%), with only 1–3% circulating as unbound, biologically active free cortisol.
4. Circadian Rhythmicity: Cortisol secretion exhibits a striking 24-hour diurnal rhythm. Pulsatile secretion peaks sharply in the early morning between 06:00 and 08:00 AM (the "cortisol awakening response") and declines progressively throughout the daytime, reaching an absolute nadir around midnight (23:00 to 01:00 AM).

Due to dynamic circadian fluctuations and protein binding, pre-analytic compliance is paramount:

1. Suspected Adrenal Insufficiency (Hypocortisolemia):

• Unexplained fatigue, weakness, anorexia, unexplained weight loss.
• Postural hypotension, refractory shock in sepsis, recurrent hypoglycemia.
• Hyponatremia with hyperkalemia, cutaneous hyperpigmentation.
• Abrupt cessation of chronic glucocorticoid therapy or pituitary surgery/radiation.

1. Suspected Cushing Syndrome (Hypercortisolemia):

• Progressive central adiposity, moon facies, buffalo hump, facial plethoric flushing.
• Wide (> 1 cm), violaceous abdominal striae, proximal muscle myopathy.
• Early-onset osteopenia/osteoporosis, spontaneous ecchymoses, difficult-to-control hypertension/type 2 diabetes.

1. Evaluation of Adrenal Incidentaloma: Autonomous cortisol secretion screening in all incidentally discovered adrenal nodules.

Step 1: Check 08:00 AM Morning Serum Cortisol
   ├─ Cortisol < 140 nmol/L (< 5 ug/dL) ───> HIGH probability of Adrenal Insufficiency ──> Confirm with Synacthen Test
   ├─ Cortisol 140 - 500 nmol/L (Gray Zone) > Indeterminate ──────────────────────────────> Must perform 250 ug Synacthen Test
   └─ Cortisol > 500 nmol/L (> 18 ug/dL) ──> NORMAL adrenal reserve ─────────────────────> Adrenal insufficiency excluded

Step 2: Perform 250 ug Cosyntropin (Synacthen) Stimulation Test
   ├─ Peak Cortisol < 500 nmol/L (< 18 ug/dL) at 30 or 60 min ──> Confirmed Adrenal Insufficiency ──> Proceed to Step 3
   └─ Peak Cortisol >= 500 nmol/L (>= 18 ug/dL) ────────────────> Normal Cortisol Reserve (Excluded)

Step 3: Measure Baseline Plasma ACTH (Draw simultaneously at Step 1)
   ├─ ACTH Markedly ELEVATED (> 2-fold ULN, often > 100 pg/mL) ──> PRIMARY Adrenal Insufficiency (Addison Disease)
   │     └─ Etiology: Autoimmune adrenalitis (check anti-21-hydroxylase), TB, adrenal hemorrhage
   └─ ACTH LOW or Inappropriately NORMAL (< 10-20 pg/mL) ────────> SECONDARY / TERTIARY Adrenal Insufficiency
         └─ Etiology: Prior exogenous steroids, pituitary adenoma, Sheehan syndrome, craniopharyngioma

Step 1: Exclude Exogenous Glucocorticoids (Oral, inhaled, topical, intra-articular)

Step 2: Perform First-Line Screening Tests (Need >= 2 Concordant Abnormal Tests)
   ├─ Overnight 1 mg Dexamethasone Suppression: Post-dex 08:00 cortisol >= 50 nmol/L (1.8 ug/dL)
   ├─ Late-Night Salivary Cortisol: Two elevated midnight collections (> 4 nmol/L)
   └─ 24-Hour Urinary Free Cortisol: Elevated > 3x upper limit of normal

Step 3: Measure Baseline Plasma ACTH to Establish Dependency
   ├─ ACTH < 5-10 pg/mL (Suppressed) ─────> ACTH-INDEPENDENT Cushing Syndrome (Order Adrenal CT/MRI)
   └─ ACTH > 15-20 pg/mL (Elevated/Normal) > ACTH-DEPENDENT Cushing Syndrome ──> Proceed to Step 4

Step 4: Differentiate Pituitary vs Ectopic ACTH
   ├─ High-Dose (8 mg) Dex Suppression / CRH Stimulation / Pituitary MRI / Inferior Petrosal Sinus Sampling (IPSS)
   ├─ Pituitary Cushing Disease (80%): Suppresses on high-dose dex, central-to-peripheral IPSS gradient > 2:1
   └─ Ectopic ACTH Source (20%): SCLC, bronchial/thymic/pancreatic neuroendocrine tumor; zero suppression

• Pathology: Bilateral destruction of all three zones of the adrenal cortex.
• Biochemical Profile: Low cortisol, low aldosterone, markedly high plasma ACTH, markedly high plasma renin.
• Electrolytes: Hyponatremia with hyperkalemia, non-anion gap metabolic acidosis, hypoglycemia.
• Physical Clues: Hyperpigmentation of gingiva, palmar creases, and surgical scars (due to excess POMC/α-MSH).

• Pathology: Deficient pituitary ACTH (secondary) or hypothalamic CRH (tertiary, commonest due to chronic therapeutic steroid suppression).
• Biochemical Profile: Low cortisol, low/normal ACTH.
• Preserved Aldosterone: The mineralocorticoid pathway in the zona glomerulosa is stimulated by angiotensin II and potassium, NOT ACTH. Therefore, aldosterone is preserved, and hyperkalemia is characteristically ABSENT!
• No Hyperpigmentation: ACTH levels are low, so skin pigmentation is pale, not dark.

• Cushing Disease: Pituitary microadenoma secreting ACTH. Partially retains negative feedback: fails to suppress on 1 mg dex, but suppresses on high-dose (8 mg) dexamethasone; stimulates on CRH; central/peripheral IPSS gradient > 2:1 (baseline) or > 3:1 (post-CRH).
• Ectopic ACTH (e.g. Small Cell Lung Carcinoma): Autonomous non-pituitary ACTH production. Refractory to high-dose dexamethasone; no IPSS gradient; presents with severe hypokalemic metabolic alkalosis, profound muscle wasting, and dark hyperpigmentation.

1. Oral Estrogens and OCPs: Dramatically increase hepatic synthesis of Corticosteroid-Binding Globulin (CBG). Total morning serum cortisol can double (700–1100 nmol/L), producing false-positive ODST suppression tests. Free cortisol (salivary or urine) is unaffected. Stop estrogens for 6 weeks before testing.
2. CYP3A4 Inducers (Phenytoin, Carbamazepine, Rifampin, St. John's Wort): Markedly accelerate the hepatic clearance of dexamethasone. During the 1 mg ODST, serum dexamethasone falls below therapeutic levels by morning, leading to false-positive failure of suppression.
3. Synthetic Glucocorticoid Cross-Reactivity in Cortisol Immunoassays:

• Prednisone / Prednisolone / Hydrocortisone: Cross-react strongly with standard cortisol antibodies, producing falsely elevated cortisol.
• Dexamethasone: Has zero cross-reactivity with cortisol immunoassays (which is why dexamethasone is uniquely utilized for suppression testing).

• Random Daytime Cortisol is Clinically Useless: Ordering a "cortisol" at 14:00 or 18:00 without dynamic stimulation or suppression testing is completely uninterpretable due to wide diurnal fluctuations and episodic secretory spikes.
• Critical Illness and Loss of Rhythm: In septic shock or polytrauma, normal circadian rhythm is abolished. A morning cortisol of 300 nmol/L in an outpatient is reassuring, but in septic shock requiring noradrenaline, a cortisol < 500 nmol/L indicates relative adrenal insufficiency (CIRCI).
• Adrenal Incidentalomas: 5–10% of elderly adults have benign non-functioning adrenal adenomas on CT. Never order abdominal CT before biochemical testing has proven hypercortisolemia!

> [!CAUTION] > Immediate Medical Emergency: Acute Adrenal Crisis: > - Presentation: Refractory hypotension/shock resistant to vasopressors and crystalloids, acute severe abdominal pain mimicking an acute surgical abdomen, vomiting, hypoglycemia, and altered consciousness. > - Management: > 1. Draw blood immediately for STAT 08:00 cortisol and ACTH. > 2. DO NOT DELAY TREATMENT waiting for lab results or dynamic cosyntropin tests! > 3. Administer IV Hydrocortisone $100\text{ mg}$ STAT, followed by 50–100 mg IV every 6 hours. > 4. Infuse rapid IV isotonic saline (0.9% NaCl) with 5%–10% Dextrose.

• Short Synacthen Test Diagnostic Rule: Peak cortisol ≥ 500 nmol/L (≥ 18 µg/dL) at 30 or 60 minutes rules out primary adrenal insufficiency with > 95% accuracy.
• Why Cosyntropin Test is Abnormal in Chronic Secondary Adrenal Insufficiency: In long-standing pituitary ACTH deficiency (> 4–6 weeks), the adrenal cortex undergoes anatomical disuse atrophy. When synthetic ACTH is injected, the atrophied cortex cannot respond acutely, yielding a subnormal peak cortisol despite an intact primary adrenal gland.
• 21-Hydroxylase Autoantibodies: Present in > 90% of patients with newly diagnosed autoimmune Addison disease.
• Inferior Petrosal Sinus Sampling (IPSS): The definitive gold standard to confirm pituitary vs ectopic source in ACTH-dependent Cushing. Central-to-peripheral ACTH ratio ≥ 2.0 at baseline or ≥ 3.0 after peripheral desmopressin/CRH confirms pituitary Cushing disease.

• Serum Electrolytes: Classic Addison disease pattern shows hyponatremia with hyperkalemia and mild metabolic acidosis.
• Complete Blood Count (CBC): Adrenal insufficiency often reveals relative lymphocytosis, normocytic anemia, and peripheral eosinophilia.
• Thyroid Function Tests (TFT): In secondary adrenal insufficiency, always evaluate concurrent central hypothyroidism (TSH and Free T4). Critical Rule: In panhypopituitarism, ALWAYS replace glucocorticoids BEFORE starting levothyroxine to prevent precipitating an acute fatal adrenal crisis!
• Glucose / Glycemic Panel: Unexplained fasting hypoglycemia in an adult should prompt immediate HPA axis evaluation.