Explaining the Cosyntropin Stimulation Test to a Patient
The cosyntropin test is a simple blood test that checks if your adrenal glands can produce enough cortisol, a vital stress hormone your body needs to function properly.
What Happens During the Test
- You'll have a blood sample drawn first to measure your baseline cortisol level 1, 2.
- Then you'll receive an injection of cosyntropin (also called Synacthen or synthetic ACTH), which is a synthetic version of a hormone that normally tells your adrenal glands to make cortisol 3.
- The standard dose is 0.25 mg (250 micrograms), given either into a vein or muscle 1, 3.
- Additional blood samples are drawn 30 and 60 minutes after the injection to see how much cortisol your adrenal glands produced in response 1, 2.
What the Test Tells Your Doctor
- If your cortisol level rises appropriately (typically above 500 nmol/L or about 14-15 μg/dL depending on the specific lab test used), your adrenal glands are working normally 1, 2, 4, 5.
- If your cortisol level stays low (below 500 nmol/L), this indicates adrenal insufficiency—meaning your adrenal glands cannot produce enough cortisol 1, 2.
- Your doctor will also check your ACTH level to determine whether the problem is in the adrenal glands themselves (primary adrenal insufficiency) or in the pituitary gland that controls them (secondary adrenal insufficiency) 1, 2.
Important Things to Know Before the Test
Medications That Can Affect Results
- Stop glucocorticoids and spironolactone on the day of testing, as these can falsely elevate your cortisol levels 3.
- Stop estrogen-containing medications 4-6 weeks before the test if possible, as estrogen increases cortisol-binding proteins and can make results inaccurate 3.
- Long-acting steroids like dexamethasone may need to be stopped for longer periods before testing 3, 6.
Timing Considerations
- The test can be performed at any time of day, though morning testing is often preferred 7.
- If you've recently started high-dose steroid therapy, the test should ideally be done within the first 3 days to get accurate results 6.
Safety and Side Effects
- The test is very safe for most people 3.
- Allergic reactions are rare but possible—tell your doctor immediately if you develop rash, hives, facial swelling, chest tightness, or wheezing during or after the test 3.
- The test is safe during pregnancy and should not be delayed if adrenal insufficiency is suspected, as undiagnosed adrenal insufficiency can be dangerous for both mother and baby 3.
What Happens After the Test
- Your doctor will review your cortisol response along with your ACTH level and clinical symptoms 1, 2.
- The specific cutoff values used to interpret your results depend on which laboratory assay your hospital uses—newer tests typically use lower thresholds (around 14-15 μg/dL) compared to older tests (18 μg/dL) 4, 5.
- If the test shows adrenal insufficiency, you'll need further evaluation to determine the cause and will likely require cortisol replacement therapy 1.
Common Pitfalls Your Doctor Will Consider
- Very low baseline cortisol (below 2 μg/dL) is highly predictive of adrenal insufficiency even before the stimulation test 5.
- Critical illness, sepsis, and malnutrition can affect how your body responds to the test 8, 9.
- Your doctor must interpret the results in the context of your overall clinical picture, not just the numbers alone 7.