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Cosyntropin

Written by The Peptide Record editorialClinical review not yet completedLast reviewed 15 September 2026

Cosyntropin (Cortrosyn) is a real, FDA-approved synthetic ACTH fragment used purely as a diagnostic tool — injected to test whether the adrenal glands can respond to a stimulus, not as a treatment for anything.

On this page
  1. A fragment, not the full hormone
  2. How the test actually works
  3. What it isn't for
  4. Current status

In brief

Should you care? Relevant if you or your doctor are investigating adrenal insufficiency — this is a diagnostic tool, not a therapy.

The short version

  • A 24-amino-acid fragment of the full 39-amino-acid natural ACTH molecule — the active portion, without most of the immunologic baggage of the full hormone.
  • In clinical use since around 1970 as the standard "ACTH stimulation test."
  • A normal test doesn't fully rule out early secondary adrenal insufficiency — a real diagnostic limitation worth knowing.

A fragment, not the full hormone

Cosyntropin is synthetic ACTH(1-24) — the biologically active N-terminal 24-amino-acid fragment of the full 39-amino-acid natural corticotropin (ACTH) molecule, which this site covers in its full-length, animal-extracted form as Corticotropin (Acthar Gel). Full corticosteroidogenic activity resides in this shorter N-terminal fragment, and it carries markedly less immunologic/antigenic baggage than the full-length hormone — a meaningfully different product from Acthar Gel, not just a generic version of it.

How the test actually works

The "cosyntropin stimulation test" (or ACTH stim test) measures serum cortisol before and 30-60 minutes after injecting a standard 250mcg dose (a "low-dose" 1mcg protocol is also used in some settings). A normal response — peak cortisol reaching roughly 18mcg/dL or higher — indicates the adrenal glands can respond appropriately and generally rules out adrenal insufficiency. A blunted response, where cortisol fails to reach that threshold, indicates adrenal insufficiency. One real diagnostic limitation worth knowing: the test doesn't by itself distinguish primary adrenal insufficiency (the gland itself can't respond) from secondary/tertiary adrenal insufficiency, and in early secondary adrenal insufficiency — before the adrenal glands have had time to atrophy from lack of stimulation — the test can come back falsely normal.

What it isn't for

Cosyntropin has no therapeutic use — it exists purely to provoke a diagnostic response, administered once (or occasionally repeated for monitoring), not as an ongoing treatment. It has no history of grey-market or bodybuilding-adjacent misuse.

Current status

Brand Cortrosyn is currently manufactured by Amphastar Pharmaceuticals; generic cosyntropin is also available. Still in routine clinical/diagnostic use, not discontinued.

References

  1. FDA approval and clinical use history for cosyntropin (Cortrosyn) as a diagnostic ACTH-stimulation agent, in use since approximately 1970.