Diagnosing ACTH-Dependent Cushing's Syndrome

Diagnostic Criteria for ACTH-Dependency

• The Endocrine Society recommends that ACTH levels >5 pg/mL are detectable and suggest ACTH-dependent Cushing's syndrome with high certainty 1
• ACTH levels >29 pg/mL have 70% sensitivity and 100% specificity for diagnosing Cushing's disease (pituitary source of ACTH) 1, 2
• Morning plasma ACTH levels combined with elevated cortisol and failed low-dose dexamethasone suppression test (LDDST) establish the diagnosis of ACTH-dependent Cushing's syndrome 2

Diagnostic Algorithm

• The American College of Endocrinology recommends confirming hypercortisolism using at least two first-line tests: 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), and low-dose dexamethasone suppression test (LDDST) 2
• ACTH levels >5 pg/mL indicate ACTH-dependent Cushing's syndrome, while ACTH levels <5 pg/mL suggest ACTH-independent (adrenal) Cushing's syndrome 1, 2

Important Clinical Considerations

• The European Society of Endocrinology notes that ACTH-dependent Cushing's syndrome accounts for 75-80% of pediatric cases and is most commonly caused by pituitary adenomas 3
• In adults with Cushing's syndrome, ACTH-dependent forms are also predominant, according to the American Association of Clinical Endocrinologists 1

Further Diagnostic Steps

• For confirmed ACTH-dependent Cushing's syndrome, pituitary MRI is recommended as the next step by the Endocrine Society 1, 2
• Bilateral inferior petrosal sinus sampling (BIPSS) has diagnostic criteria of central-to-peripheral ACTH ratio ≥2:1 before CRH stimulation and ≥3:1 after stimulation, with sensitivity of 100% 2