What is the normal adult ACTH (adrenocorticotropic hormone) level and how should the blood sample be drawn for accurate measurement?

Normal ACTH Reference Range and Specimen Collection

The normal morning (08:00-09:00h) plasma ACTH reference range is approximately 10-60 pg/mL, with any detectable level >5 pg/mL considered physiologically present. 1

Reference Values and Diagnostic Thresholds

Morning ACTH levels should be interpreted as follows:

• Normal range: 10-60 pg/mL when measured at 08:00-09:00h 1
• Detectable threshold: >5 pg/mL indicates measurable ACTH and suggests ACTH-dependent disease in the setting of hypercortisolism 2, 1
• Cushing's disease threshold: >29 pg/mL provides 70% sensitivity and 100% specificity for pituitary Cushing's disease when hypercortisolism is confirmed 2, 1
• Suppressed/undetectable: <5 pg/mL suggests ACTH-independent disease (adrenal source) 2, 1

Historical data from early radioimmunoassays showed normal subjects averaged 22 μμg/mL (equivalent to approximately 22 pg/mL) at 8-10 AM and 9.6 μμg/mL in the late evening, confirming the circadian rhythm. 3

Critical Specimen Collection Requirements

ACTH is extremely unstable and requires meticulous handling to prevent degradation and false-low results. 1

Timing

• Collect at 08:00-09:00h for standardization and comparison with established diagnostic cutoffs 2, 1
• Morning collection captures the physiologic peak of the circadian rhythm 2
• Do NOT collect in the afternoon or evening, as ACTH levels are physiologically lower and established diagnostic thresholds do not apply 2

Sample Handling

• Use EDTA (purple-top) tubes for plasma collection 1
• Place specimen on ice immediately after collection 1
• Process and separate plasma within 15 minutes to prevent enzymatic degradation 1
• ACTH has a plasma half-life of only 22-30 minutes, making rapid processing essential 3

Patient Preparation

• No fasting is required for ACTH measurement 2
• Verify the patient is NOT taking exogenous steroids (oral prednisolone, dexamethasone, inhaled fluticasone), as these suppress ACTH and invalidate interpretation 4, 5, 1
• Ensure the patient is not acutely stressed during collection, as stress appropriately elevates ACTH 1

Interpretation in Clinical Context

ACTH must ALWAYS be interpreted alongside simultaneous cortisol levels—never in isolation. 1

Primary Adrenal Insufficiency

• Low cortisol (<250 nmol/L) + elevated ACTH is diagnostic 4, 1
• The elevated ACTH reflects appropriate pituitary compensation for adrenal failure 4

Secondary Adrenal Insufficiency

• Low cortisol + inappropriately normal or low ACTH indicates pituitary or hypothalamic dysfunction 1

ACTH-Dependent Cushing's Syndrome

• Elevated cortisol + ACTH >5 pg/mL indicates either pituitary Cushing's disease or ectopic ACTH secretion 2, 1
• ACTH >29 pg/mL strongly favors pituitary disease (70% sensitivity, 100% specificity) 2

ACTH-Independent Cushing's Syndrome

• Elevated cortisol + ACTH <5 pg/mL or undetectable indicates an adrenal source (adenoma, carcinoma, or hyperplasia) 2, 1

Common Pitfalls to Avoid

• Delayed processing: ACTH degrades rapidly at room temperature, producing falsely low results 1
• Wrong tube type: Serum (red-top) tubes are inappropriate; only EDTA plasma is acceptable 1
• Afternoon collection: Physiologically low evening ACTH may be misinterpreted as pathologic suppression 2
• Ignoring exogenous steroids: Any glucocorticoid use (including inhaled or topical) suppresses ACTH and confounds interpretation 4, 5, 1
• Interpreting ACTH without cortisol: A "normal" ACTH may be inappropriately low for a low cortisol (secondary insufficiency) or inappropriately high for an elevated cortisol (Cushing's disease) 1
• Cyclic disease: In cyclic Cushing's syndrome, ACTH must be measured during documented hypercortisolism to avoid false-negative results 2, 1