What it is
Cosyntropin is the front 24 amino acids of ACTH, made synthetically. Natural ACTH, the pituitary's cortisol signal, is 39 residues long (see the ACTH entry); cosyntropin stops at residue 24:
SYSMEHFRWGKPVGKKRRPVKVYP
The Cortrosyn label gives its formula as C136H210N40O31S and its weight as 2,934 g/mol. Each single-dose vial holds 0.25 mg of powder with mannitol. Outside the US the same molecule is known as tetracosactide, sold in Europe as Synacthen (PubChem synonyms, 2026-10-05).
Why only 24 residues
The label explains why the tail can go. All of ACTH's adrenal-stimulating activity sits at the front: residues 1–20 are the shortest sequence with full activity, and cutting from 20 to 19 loses 70% of potency. Most of its ability to provoke antibodies sits at the back, residues 22–39. A 1–24 peptide therefore keeps the biology and drops most of the immune target — the property the label singles out "in view of the known antigenicity of natural ACTH."
On potency, the label states that 0.25 mg stimulates the adrenal cortex maximally and to the same extent as 25 units of natural ACTH.
The test, as the label describes it
Cosyntropin has one approved job: standing in for ACTH in the stimulation test, which asks whether the adrenal glands can raise cortisol when told to.
| Step (Cortrosyn label) | Detail |
|---|---|
| Dose | 0.25 mg, ages 2–17 and adults; 0.125 mg, birth to under 2 years |
| Route | Intravenous or intramuscular |
| Samples | Cortisol at baseline, then exactly 30 and 60 minutes after |
| Reading | Stimulated cortisol below 18 mcg/dL at 30 or 60 minutes is "suggestive of adrenocortical insufficiency" |
| Caveat | Cut-offs "may vary according to the assay used" |
The label calls the test a screen, used in combination with other diagnostic tests. It does not, on its own, say which kind of adrenal insufficiency is present or what causes it; that is the ordering clinician's work.
What skews the result
The label's main warning is not about harm but about a wrong answer:
- Glucocorticoids and spironolactone can falsely raise measured cortisol. The label sets stopping them on the day of the test, and longer for long-acting glucocorticoids.
- Oestrogen-containing drugs raise cortisol-binding globulin, which raises total cortisol. The label sets stopping them four to six weeks before.
- Any condition that raises or lowers that binding globulin moves total cortisol with it.
Safety
Because it is given once, the label's adverse-reaction list comes from postmarketing reports rather than trials: anaphylaxis, slow or fast heart rate, high blood pressure, peripheral swelling and rash. The warnings section adds monitoring for hypersensitivity.
Where it stands
Drugs@FDA lists Cortrosyn (Amphastar, NDA 016750, 1970-04-22) and a Sandoz generic (ANDA 202147, 2012-06-29) as prescription products; a Sandoz new-drug application (2008) and another generic (2009) are discontinued (openFDA, 2026-10-05). PubMed indexes 2,230 records for cosyntropin and 133 tagged as randomised controlled trials (2026-10-05) — a count that mostly reflects its use as a measuring tool inside trials of other treatments, not trials of cosyntropin itself.
Related reading
The full hormone, its precursor and the porcine gels sold as treatments are in the ACTH entry. ACTH is cut from the same precursor as the melanocyte-stimulating hormones that melanotan-1 and melanotan-2 were modelled on.