The diagnosis of Addison’s disease relies on a series of specialized blood tests designed to measure the levels of hormones produced by the adrenal glands and the brain. Because the symptoms of adrenal insufficiency are often non specific and develop slowly over time, doctors use these biochemical markers to confirm whether the adrenal glands are physically capable of meeting the body’s hormonal demands. In the United Kingdom, the diagnostic pathway typically begins with a simple morning blood test and progresses to more complex dynamic stimulation tests if the initial results are inconclusive. Accurate diagnosis is essential because the treatment involves lifelong hormone replacement, and a delay in identifying the condition can lead to a medical emergency.
What We’ll Discuss in This Article
- The role of the random and morning cortisol blood test in initial screening.
- The Synacthen stimulation test as the gold standard for confirming adrenal failure.
- How measuring plasma ACTH levels distinguishes between primary and secondary causes.
- The significance of aldosterone and renin tests in managing salt balance.
- Identifying adrenal antibodies to confirm an autoimmune trigger.
- Frequently asked questions regarding the preparation and accuracy of these tests.
The Short Synacthen Stimulation Test
The Synacthen stimulation test, also known as the ACTH stimulation test, is the most reliable method for diagnosing Addison’s disease. This test evaluates the reserve capacity of the adrenal glands by simulating a high stress situation. During the procedure, a baseline blood sample is taken, and then the patient is given an injection of Synacthen, which is a synthetic version of the adrenocorticotropic hormone (ACTH) normally produced by the pituitary gland. Further blood samples are then taken, typically 30 minutes and sometimes 60 minutes after the injection, to measure how much the cortisol levels have risen.
In a healthy individual, the injection of Synacthen should trigger the adrenal glands to produce a significant surge in cortisol. According to the NICE guidance on adrenal insufficiency, a normal response is generally defined as a peak cortisol level of at least 420 to 450 nmol/L (depending on the specific laboratory reference range). If the adrenal glands are damaged, as they are in Addison’s disease, they will show little to no response to the injection, and the cortisol levels will remain low. This lack of response confirms that the adrenal glands are unable to function correctly, providing a definitive diagnosis of primary adrenal insufficiency.
Distinguishing Primary and Secondary Causes
Once adrenal insufficiency is confirmed by the Synacthen test, doctors must determine if the problem lies in the adrenal glands themselves (primary) or in the pituitary gland in the brain (secondary). This is achieved by measuring the level of plasma ACTH in the blood. ACTH is the signal the brain sends to the adrenals; in a healthy system, if cortisol is low, the brain should produce very high amounts of ACTH to try and fix the problem.
In primary Addison’s disease, the ACTH level will be significantly elevated, often exceeding 200 ng/L, because the pituitary gland is working overtime to stimulate the failing adrenals. In secondary adrenal insufficiency, the ACTH level will be inappropriately low or within the “normal” range despite the low cortisol, indicating that the pituitary gland is failing to send the necessary signal. This distinction is vital because primary Addison’s disease requires the replacement of both cortisol and aldosterone, whereas secondary insufficiency usually only requires cortisol replacement.
Testing for Adrenal Antibodies
To determine the underlying cause of the adrenal failure, doctors in the UK will often request a test for adrenal antibodies, specifically the 21 hydroxylase antibody. As discussed in previous sections, the majority of cases of Addison’s disease in the United Kingdom are autoimmune in origin. The presence of these antibodies in the blood provides strong evidence that the immune system is responsible for the damage to the adrenal glands.
A positive antibody test is highly specific for autoimmune Addison’s disease and helps rule out other rarer causes like tuberculosis or genetic conditions. However, it is important to note that about 10 percent of people with autoimmune adrenal failure may test negative for these antibodies, especially if the disease has been present for a long time and the immune response has quieted down. If the antibody test is negative and there is no other obvious cause, doctors may proceed with imaging tests, such as a CT scan of the abdomen, to check for signs of infection or other structural issues in the adrenal glands.
| Test Type | Expected Result in Addison’s | Clinical Meaning |
| Morning Cortisol | Low (typically <100 nmol/L) | Suggests adrenal failure |
| Synacthen Response | Blunt (peak <420 nmol/L) | Confirms adrenal failure |
| Plasma ACTH | Very High (>200 ng/L) | Confirms primary (Addison’s) |
| Plasma Renin | High | Indicates mineralocorticoid deficiency |
| Adrenal Antibodies | Often Positive | Suggests autoimmune cause |
Preparation and Factors Affecting Results
To ensure the accuracy of these blood tests, patients must follow specific preparation guidelines. Most notably, many types of steroid medications can interfere with the results. For example, prednisolone can be detected as cortisol in some lab tests, leading to a falsely “normal” result. Patients are typically advised to omit their morning dose of hydrocortisone on the day of a Synacthen test or to stop taking other steroids for at least 24 hours prior to the procedure.
Oestrogen containing medications, such as the combined oral contraceptive pill or hormone replacement therapy (HRT), can also affect the accuracy of cortisol measurements. These medications increase the levels of cortisol binding globulin in the blood, which can make the total cortisol level appear higher than it truly is. For this reason, UK clinical guidelines often recommend that patients stop taking oestrogen based medications for six weeks before undergoing definitive adrenal testing, provided it is safe for them to do so and alternative contraception is in place.
Conclusion
Diagnosing Addison’s disease involves a structured approach using morning cortisol levels, the Synacthen stimulation test, and measurements of ACTH, renin, and aldosterone. These tests allow doctors to confirm the presence of adrenal insufficiency, distinguish between primary and secondary causes, and identify the underlying trigger, such as an autoimmune response. Accurate interpretation of these results is the foundation of a safe and effective treatment plan. Because the diagnostic process can be complex and influenced by various medications, it is essential to follow all preparation instructions provided by your endocrinology team. If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Do I need to fast before my Addison’s blood test?
Fasting is not usually required for a morning cortisol or Synacthen test, but you should check with your specific clinic for their instructions.
Why must the cortisol test be done so early in the morning?
Cortisol levels peak naturally between 8 am and 9 am; testing at this time provides the best opportunity to see if the glands are working correctly.
Can the Synacthen test be done if I am pregnant?
Pregnancy increases cortisol levels naturally, making the test harder to interpret, so it should be discussed with an obstetrician and endocrinologist.
What happens if the Synacthen test is borderline?
If results are equivocal, your doctor may repeat the test in a few weeks or perform a different type of stimulation test, such as an insulin tolerance test.
Does a normal cortisol test rule out Addison’s disease forever?
No, it rules out the condition at that specific moment, but if symptoms persist or worsen, the tests may need to be repeated in the future.
Will I get the results of the Synacthen test immediately?
No, the blood samples must be processed in a laboratory, and results typically take a few days to a week to be returned to your consultant.
Can I take my usual medications before the test?
You should take all your normal medicines EXCEPT for steroid tablets, as these can invalidate the results. Always confirm this with your doctor.
Authority Snapshot (E-E-A-T Block)
The purpose of this article is to explain the medical procedures and biochemical markers used to diagnose adrenal insufficiency in the UK. The content has been written by Dr Rebecca Fernandez and reviewed by Dr Stefan, physicians with extensive experience in internal medicine and endocrinology. All information is strictly aligned with the clinical diagnostic standards provided by the NHS and the latest NICE guidelines to ensure maximum accuracy for patients.
