What laboratory abnormalities are typical in Cushing’s disease caused by an ACTH (adrenocorticotropic hormone)‑secreting pituitary adenoma?

Laboratory Abnormalities in Cushing's Disease

In Cushing's disease caused by an ACTH-secreting pituitary adenoma, expect elevated or normal-high ACTH levels (>5 pg/mL, often >29 pg/mL), elevated 24-hour urinary free cortisol (>100 μg/24h), elevated late-night salivary cortisol (>3.6 nmol/L), loss of normal circadian rhythm with elevated midnight cortisol (>50 nmol/L), and failure to suppress cortisol below 1.8 μg/dL on the 1-mg overnight dexamethasone suppression test. 1, 2, 3

Core Biochemical Features

ACTH Levels

• Morning plasma ACTH >5 pg/mL (>1.1 pmol/L) is detectable in all patients with Cushing's disease and indicates ACTH-dependent hypercortisolism. 1, 3
• ACTH >29 pg/mL (>6.4 pmol/L) provides 70% sensitivity and 100% specificity for diagnosing pituitary Cushing's disease specifically (versus ectopic ACTH). 1, 3
• ACTH should be measured in the morning (08:00-09:00h) when levels are physiologically highest, as this timing corresponds to established diagnostic cutoffs. 1
• In contrast, ACTH-independent adrenal causes show suppressed or undetectable ACTH (<5 pg/mL). 1, 3

Cortisol Abnormalities

Loss of Circadian Rhythm:

• Midnight serum cortisol >50 nmol/L (>1.8 μg/dL) or late-night salivary cortisol >3.6 nmol/L indicates loss of the normal cortisol nadir—a hallmark of Cushing's disease. 1, 2
• Late-night salivary cortisol has 92-100% sensitivity and 93-100% specificity; obtain 2-3 samples on different nights to account for cyclic disease. 2
• A single midnight serum cortisol >207 nmol/L (>7.5 μg/dL) provides 96% sensitivity and 100% specificity. 2

24-Hour Urinary Free Cortisol:

• Values >100 μg/24h (>1.6 μmol/24h) on multiple collections (at least 2-3) are diagnostic in symptomatic patients. 2, 3
• UFC has >90% sensitivity but shows up to 50% random variability between collections, necessitating multiple measurements. 2
• Ensure complete 24-hour collection by measuring total volume and creatinine excretion; incomplete collections are the most common cause of false results. 2

Dexamethasone Suppression Test:

• Cortisol ≥1.8 μg/dL (≥50 nmol/L) at 08:00h after 1 mg dexamethasone given at 23:00-24:00h indicates abnormal suppression. 2, 3
• Cortisol >5 μg/dL (>138 nmol/L) post-dexamethasone indicates overt Cushing's syndrome. 2
• Measure dexamethasone levels concomitantly to exclude false-positive results from abnormal drug metabolism or malabsorption. 1, 2

Additional Laboratory Findings

Metabolic Abnormalities

• Hypokalemia may be present but is more pronounced in ectopic ACTH syndrome; when severe, it suggests extremely high cortisol levels overwhelming 11β-HSD2 and producing mineralocorticoid effects. 1
• Hyperglycemia and impaired glucose tolerance are common due to cortisol's anti-insulin effects. 4
• Hyponatremia is less common in Cushing's disease but may occur. 2

Pattern Recognition

• The coefficient of variation of cortisol levels is typically subnormal in Cushing's disease, reflecting altered pulsatility. 5
• Some patients retain a circadian rhythm pattern but with elevated baseline levels throughout the 24-hour period. 5
• Cushing's disease often shows a combination of episodic cortisol release (under residual CRH control) and continuous secretion from autonomous ACTH. 5

Critical Diagnostic Pitfalls

False Elevations to Exclude

• Oral estrogen/contraceptives increase cortisol-binding globulin (CBG), falsely elevating total cortisol while free cortisol remains normal—always inquire about these before pursuing extensive workup. 2
• Pregnancy similarly increases CBG and total cortisol. 2
• CYP3A4 inducers (phenytoin, rifampin, carbamazepine) accelerate dexamethasone metabolism, causing false-positive suppression tests. 2
• Exogenous glucocorticoids (oral, inhaled fluticasone, topical, intra-articular/epidural injections) must be excluded before any testing, as they suppress endogenous cortisol and invalidate results. 2

Pseudo-Cushing's States

• Severe obesity, depression, alcoholism, and polycystic ovary syndrome can activate the HPA axis and cause mildly elevated cortisol that mimics true hypercortisolism. 2, 3
• These conditions typically show blunted ACTH responses to CRH stimulation, unlike true Cushing's disease. 1

Cyclic Cushing's Disease

• Patients may have weeks to months of normal cortisol secretion interspersed with hypercortisolism, producing inconsistent test results depending on sampling timing. 1, 2
• This necessitates repeated testing (2-3 measurements of each modality) and extended monitoring during symptomatic periods. 2

Diagnostic Algorithm Summary

1. Screen with 2-3 measurements of at least two tests: 24-hour UFC, late-night salivary cortisol, and/or 1-mg overnight dexamethasone suppression test. 2, 3
2. Measure morning (09:00h) plasma ACTH once hypercortisolism is confirmed to determine ACTH-dependency. 1, 3
3. ACTH >5 pg/mL confirms ACTH-dependent disease; proceed to pituitary MRI. 1, 3
4. If MRI shows adenoma ≥10 mm, presume Cushing's disease and proceed to surgery. 1
5. If MRI shows lesion 6-9 mm, perform CRH or desmopressin stimulation test (cortisol rise >38 nmol/L at 15 minutes supports pituitary source). 1
6. If MRI shows no adenoma or lesion <6 mm, perform bilateral inferior petrosal sinus sampling (BIPSS) with central-to-peripheral ACTH ratio ≥2:1 baseline or ≥3:1 post-stimulation confirming pituitary source. 1, 3