Endocrine dynamic function tests - ACTH stimulation test (short synacthen test)

Role

The short ACTH (Synacthen) stimulation test assesses adrenal cortisol reserve by measuring the cortisol response to synthetic ACTH administration - the standard confirmatory test for suspected adrenal insufficiency, whether primary (adrenal) or secondary (pituitary/hypothalamic) in origin
The standard diagnostic test whenever adrenal insufficiency is clinically suspected, given its practicality (can be performed at any time of day, unlike some other dynamic tests requiring specific timing)

Synthetic ACTH directly stimulates adrenal cortisol production - an inadequate cortisol rise indicates insufficient adrenal reserve, whether from primary adrenal gland damage (Addison disease) or from chronic ACTH deficiency causing adrenal atrophy (secondary insufficiency, where the adrenal gland itself is intrinsically normal but under-stimulated over time)

Baseline cortisol is measured, synthetic ACTH is administered, and cortisol is re-measured at a fixed interval (commonly 30 minutes) - an adequate peak cortisol response excludes significant adrenal insufficiency, while an inadequate response confirms it
Very recent-onset secondary adrenal insufficiency (e.g. immediately following pituitary surgery or acute pituitary injury) can produce a falsely normal short Synacthen test result, since the adrenal gland has not yet had time to atrophy from reduced ACTH stimulation - clinical correlation and, if suspicion remains, alternative/repeat testing after a longer interval is needed in this specific scenario

Confirmed adrenal insufficiency -> glucocorticoid replacement (with mineralocorticoid replacement also required in primary adrenal insufficiency, but generally not needed in secondary insufficiency where the renin-angiotensin-aldosterone axis is intact)
Further testing (ACTH level, adrenal autoantibodies, pituitary imaging) distinguishes primary from secondary insufficiency and identifies the underlying cause

Primary vs secondary adrenal insufficiency distinction (mineralocorticoid replacement need), false-normal result in very recent-onset secondary insufficiency (a recognised pitfall), glucocorticoid replacement dosing and stress-dose adjustment principles

The short Synacthen test can be falsely normal in very recently developed secondary adrenal insufficiency (e.g. immediately after acute pituitary injury), since the adrenal gland has not yet atrophied from reduced ACTH stimulation - this timing-dependent limitation means a normal result soon after a pituitary insult should not be treated as definitively excluding adrenal insufficiency without further clinical correlation

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