The confirmation of Cushing’s syndrome is a meticulous process that requires a series of specialized tests to prove that the body is producing too much cortisol. Because cortisol levels naturally fluctuate throughout the day and can rise temporarily due to stress or illness, a single blood test is rarely enough to provide a definitive answer. UK clinicians follow a structured diagnostic pathway that first confirms the presence of hypercortisolism and then uses advanced imaging to locate the specific source of the hormone excess, whether it be in the pituitary gland, the adrenal glands, or elsewhere in the body. This dual approach ensures that the treatment, such as surgery or medication, is targeted accurately to restore the body’s natural hormonal balance.
What We’ll Discuss in This Article
- The three primary screening tests used to confirm high cortisol levels.
- How the dexamethasone suppression test evaluates the body’s feedback loop.
- The role of the 24 hour urinary cortisol and midnight saliva tests.
- Utilizing MRI and CT scans to locate tumours in the pituitary or adrenal glands.
- Specialized procedures like inferior petrosal sinus sampling for complex cases.
- Frequently asked questions regarding the diagnostic timeline and accuracy.
Initial Screening for High Cortisol
The first objective in the diagnostic process is to confirm that the patient’s cortisol levels are consistently higher than normal. In the United Kingdom, doctors typically use one of three main screening tests, and often a combination of two is required for a firm confirmation. The most common is the overnight dexamethasone suppression test. For this, the patient takes a small dose of a synthetic steroid (dexamethasone) late at night, and their cortisol is measured the following morning. In a healthy person, this drug signals the brain to shut down cortisol production; in someone with Cushing’s syndrome, the body ignores the signal and cortisol remains high.
Another effective screening tool is the 24 hour urinary free cortisol test. The patient collects all their urine over a full day and night, which allows the laboratory to measure the total amount of cortisol produced over 24 hours rather than a single moment in time. Finally, the late night salivary cortisol test involves taking a swab of saliva at midnight. Normally, cortisol levels should be at their lowest point at this time to allow for sleep. If the saliva test shows high levels of cortisol at midnight, it is a strong indicator that the body’s natural clock (circadian rhythm) for hormone production has been disrupted.
The Role of Blood ACTH Levels
Once high cortisol is confirmed, the next vital step is to determine the “drive” behind the overproduction. This is done by measuring the level of Adrenocorticotropic Hormone (ACTH) in the blood. ACTH is the chemical messenger sent from the brain to the adrenal glands. If the blood test shows that ACTH is very low, it suggests that the problem is located directly in the adrenal glands, usually a tumour that is making cortisol independently.
If the ACTH levels are high or “inappropriately normal,” it indicates that the adrenal glands are being told to make too much cortisol by an external signal. This points the medical team toward a problem in the pituitary gland (Cushing’s disease) or a rare “ectopic” tumour elsewhere in the body, such as the lungs. Distinguishing between these paths is essential because it determines which part of the body needs to be imaged with scans. The NICE guidelines on Cushing’s syndrome emphasize that this biochemical “mapping” must be completed before any surgery is considered.
Imaging the Pituitary and Adrenal Glands
Imaging is used only after the blood and urine tests have confirmed that a hormonal problem exists. If the biochemical tests point toward the brain, an MRI scan of the pituitary gland is the gold standard. Pituitary tumours that cause Cushing’s are often very small (microadenomas), sometimes measuring only a few millimetres, so a high resolution MRI is necessary to see them clearly. Even then, about 30 percent to 40 percent of these tumours may be too small to be seen on a standard scan.
If the tests point toward the adrenal glands, a CT scan or MRI of the abdomen is used. These scans provide a clear view of both adrenal glands, allowing doctors to see if one gland contains a tumour or if both glands are enlarged. In cases where an ectopic source is suspected (high ACTH but no pituitary tumour), a CT scan of the chest and abdomen is performed to search for hidden tumours in the lungs, thymus, or pancreas. These imaging results are the final piece of the puzzle that allows a surgeon to plan the removal of the abnormal tissue.
Specialized Diagnostic Procedures
In some complex cases, standard blood tests and scans are not enough to be 100 percent certain of the source. This is particularly true when a patient has high ACTH levels but the MRI of the pituitary gland is clear. To solve this, UK specialists may perform a procedure called inferior petrosal sinus sampling (IPSS). This involves a radiologist threading small tubes (catheters) into the veins that drain the pituitary gland.
During IPSS, blood samples are taken directly from the area around the pituitary and compared to blood samples from the rest of the body. If the ACTH levels are much higher near the pituitary, it confirms that a hidden brain tumour is the cause. If they are the same, it proves the ACTH is coming from an ectopic source elsewhere. This procedure is highly accurate and is performed in specialized centres to ensure the highest level of patient safety and diagnostic certainty.
| Test Phase | Specific Method | Diagnostic Goal |
| Confirmation | 24 hour Urine / Midnight Saliva | Prove that total cortisol is high |
| Feedback Check | Dexamethasone Suppression | Show the body cannot turn off cortisol |
| Localization | Plasma ACTH Test | Determine if the cause is brain or adrenal |
| Visual Evidence | MRI Pituitary / CT Adrenals | Physically locate the tumour |
| Advanced Map | Petrosal Sinus Sampling | Confirm pituitary source in hidden cases |
Factors Affecting Test Accuracy
Several factors can interfere with the results of these complex tests, which is why doctors often repeat them to be sure. Certain medications, such as the contraceptive pill or hormone replacement therapy (HRT), can raise the levels of proteins in the blood that bind to cortisol, making the results appear falsely high. Patients are often asked to stop these medications for several weeks before testing.
Physical or emotional stress, severe depression, and excessive alcohol consumption can also cause what is known as “Pseudo-Cushing’s.” This is a state where cortisol levels are high due to external stress rather than a tumour. UK clinical guidelines suggest that these underlying issues should be addressed or stabilized before definitive testing is completed. Following all preparation instructions, such as avoiding vigorous exercise or certain foods before a saliva test, is essential for a reliable diagnosis.
Conclusion
Confirming Cushing’s syndrome requires a combination of screening tests to prove high cortisol levels and imaging to find the specific source of the problem. From 24 hour urine collections and midnight saliva swabs to high resolution MRI scans, each step is designed to build a complete picture of the body’s hormonal health. Identifying whether the excess cortisol stems from the pituitary, the adrenal glands, or an ectopic source is the only way to ensure the correct surgical or medical treatment is provided. If you experience severe, sudden, or worsening symptoms, call 999 immediately.
How long does the whole diagnostic process take?
It can take several weeks or even months, as tests often need to be repeated and imaging appointments must be scheduled and reviewed.
Can I have Cushing’s if my MRI scan is normal?
Yes, some pituitary tumours are so small they do not show up on a scan, which is why blood and urine tests are the most important part of the diagnosis.
Do I need to stay in the hospital for these tests?
Most screening tests like urine and saliva collections are done at home, but stimulation tests or sinus sampling may require a day at the hospital.
Why do I have to collect my urine for a full 24 hours?
Cortisol levels change throughout the day; a 24 hour collection gives the most accurate measure of the total amount your body is producing.
What is the “midnight” saliva test for?
It checks if your cortisol levels drop at night like they should; in Cushing’s, the “off switch” for cortisol at night does not work.
Will my doctor do a CT scan of my whole body?
Usually, only the area suspected (the brain or the abdomen) is scanned first, unless an “ectopic” tumour in the chest is suspected.
Can other illnesses make the tests look positive?
Yes, severe depression or physical illness can raise cortisol levels, which is why doctors look for consistent results over multiple tests.
Authority Snapshot (E-E-A-T Block)
The purpose of this article is to provide clear, medically accurate information about the diagnostic pathway for cortisol overproduction. The content has been written by Dr Rebecca Fernandez and reviewed by Dr Stefan, physicians with extensive experience in the UK medical system and internal medicine. All information is strictly grounded in the latest clinical guidelines provided by the NHS and NICE to ensure maximum accuracy and patient safety.
