Cosyntropin for the ACTH Stimulation Test: Reading the Adrenal Glands in an Hour

The clinical scenario

When a clinician suspects adrenal insufficiency — the body not making enough cortisol — they need a reliable way to confirm it. The standard tool is a peptide: cosyntropin (Cortrosyn, Synacthen), the synthetic active fragment of ACTH. This use case explains how the test works; it is arranged and interpreted by clinicians.

Why the mechanism fits

Cortisol is made by the adrenal glands when they are stimulated by ACTH, the pituitary hormone. Cosyntropin is the first 24 amino acids of ACTH — the part that carries all of its adrenal-stimulating activity. Injecting it is like sending the adrenal glands a maximal "make cortisol" signal and then checking whether they can answer. (For where this sits in the wider circuit, see the HPA axis.)

How the test runs

The ACTH (cosyntropin) stimulation test is quick:

1. A baseline blood cortisol is drawn.
2. A standard dose of cosyntropin is injected (intravenously or intramuscularly).
3. Cortisol is measured again at 30 and/or 60 minutes.

Healthy adrenal glands respond by pushing cortisol above a defined threshold. A blunted rise indicates adrenal insufficiency. The whole test takes about an hour.

Standard-dose vs low-dose

• Standard (250 mcg). A large, supraphysiologic dose that maximally stimulates the adrenals — the classic, widely used version.
• Low-dose (1 mcg). Some clinicians use this because they consider it more sensitive for milder or secondary (pituitary-related) insufficiency. It has trade-offs in practicality and interpretation.

The choice depends on what the clinician suspects and local practice.

Reading the result in context

A stimulation test rarely stands alone. Because in long-standing secondary insufficiency the adrenals can atrophy from lack of ACTH and also respond poorly, the result is interpreted alongside a baseline ACTH level (high in primary/Addison's, low in secondary) and, in selected cases, a corticorelin (CRH) test to probe the pituitary. The time of day matters too, because cortisol follows a daily rhythm.

An important clarification

Cosyntropin is a diagnostic, not a treatment. Adrenal insufficiency is treated by replacing the missing hormones (hydrocortisone, plus a mineralocorticoid in primary disease). The test's value is that it confirms the diagnosis quickly and safely, opening the door to that lifesaving replacement and to education about preventing adrenal crisis.

This use case is educational and not medical advice. Adrenal testing and treatment are managed by an endocrinologist.