⚠️ Emergency tip: If a patient is in extremis with suspected Addison’s disease or adrenal crisis, draw blood for cortisol and ACTH, then treat immediately with IV hydrocortisone - confirm the diagnosis later biochemically.
Never delay life-saving corticosteroid therapy for diagnostic testing.
📘 About
- Always check the BNF entry here for detailed guidance and preparation specifics.
- Tetracosactide (Synacthen®) is a synthetic analogue of adrenocorticotropic hormone (ACTH 1–24) used for diagnostic testing of adrenal function.
- It stimulates the adrenal cortex to produce cortisol, allowing assessment of adrenal reserve and pituitary–adrenal axis integrity.
⚙️ Mode of Action
- Mimics endogenous ACTH → binds to melanocortin-2 receptors on adrenal cortical cells.
- Stimulates synthesis and release of cortisol, aldosterone, and androgens from the adrenal cortex.
- The rise in plasma cortisol after administration indicates preserved adrenal responsiveness.
💊 Indications & Dosing (Short Synacthen Test)
- Purpose: Diagnosis or exclusion of primary adrenal insufficiency (Addison’s disease) or secondary adrenal suppression (e.g. pituitary disease, chronic steroid use).
- Preparation: Ideally, the patient should fast overnight and omit their morning steroid dose if applicable.
- Protocol:
- Take baseline (0 min) blood sample for cortisol (and ACTH if Addison’s suspected).
- Inject tetracosactide 250 micrograms IM (or IV).
- Repeat serum cortisol sampling at 30 minutes (and sometimes 60 minutes).
- Interpretation:
- Normal response: serum cortisol ≥ 450–550 nmol/L (depending on local assay and guidance).
- Inadequate rise: suggests adrenal insufficiency - primary if ACTH high, secondary if ACTH low.
📈 Example Diagnostic Patterns
| Condition |
Baseline Cortisol |
Post-Synacthen |
ACTH |
| Primary adrenal insufficiency (Addison’s) |
Low |
Fails to rise |
High |
| Secondary (pituitary) insufficiency |
Low |
Fails to rise |
Low or normal |
| Normal adrenal function |
Normal |
Rises appropriately |
Normal |
💥 Adverse Effects
- Minimal with single diagnostic doses.
- Rare: facial flushing, mild nausea, or transient blood pressure rise.
- Repeated administration (uncommon) may cause hypersensitivity reactions.
🧠 Teaching Note
The Short Synacthen Test is the standard dynamic assessment of adrenal function.
Remember that cortisol peaks at 30 minutes and that results must be interpreted in the context of recent steroid use, critical illness, and local assay standards.
If Addison’s is suspected clinically, never delay hydrocortisone treatment - diagnostic confirmation can follow once the patient stabilises.
📚 References
- BNF: Tetracosactide (Synacthen)
- NICE NG51: Sepsis - Recognition, Diagnosis and Early Management (adrenal crisis context)
- Society for Endocrinology: Adrenal Crisis Guidance (2023)
