Introduction
If you have been feeling unusually exhausted or dizzy, you may wonder whether cortisol is involved. A GP or specialist might mention a CRH test when investigating how your body produces this hormone, particularly if earlier results need a closer look.
A CRH stimulation test examines how hormone signals travel from the brain to the adrenal glands. A clinician gives corticotropin-releasing hormone, or CRH, through a vein and takes blood samples over time. The samples can measure adrenocorticotropic hormone, or ACTH, and cortisol. (niddk.nih.gov)
This article is for adults who have been offered a CRH test or are trying to understand its role in adrenal insufficiency, a condition in which the body does not make enough cortisol. We explain what the test can show, how it differs from an ACTH stimulation test and what to ask about your results.
Persistent or concerning symptoms deserve a GP assessment first. Keeping a record of symptoms and medicines can help that conversation, while any testing should be chosen to answer a specific clinical question.
How CRH, ACTH and cortisol work together
Cortisol is a hormone made by the adrenal glands, which sit above the kidneys. It has several roles, including helping the body respond to physical stress and regulating blood glucose and blood pressure.
Its production is controlled through a chain of signals called the hypothalamic-pituitary-adrenal axis, often shortened to the HPA axis:
- The hypothalamus, an area of the brain, releases CRH.
- CRH signals the pituitary gland, a small gland at the base of the brain, to release ACTH.
- ACTH signals the adrenal glands to produce cortisol.
Cortisol also feeds back to the brain and pituitary, helping regulate those signals. The CRH laboratory information describes this signalling pathway. (interscienceinstitute.com)
A CRH stimulation test looks at the response after a CRH signal is given. It is different from simply measuring the amount of CRH in a blood sample. It is also different from taking one cortisol sample without giving a stimulating hormone.
Why might a doctor request a CRH stimulation test?
A specialist may consider a CRH stimulation test when they need more information about the signals that control cortisol. It can be one option when adrenal insufficiency is being investigated and the results of an earlier ACTH stimulation test are not clear.
According to the National Institute of Diabetes and Digestive and Kidney Diseases, a CRH test can help assess secondary adrenal insufficiency and help distinguish it from tertiary adrenal insufficiency. (niddk.nih.gov)
These terms describe where a problem in the signalling pathway may arise:
- Primary adrenal insufficiency involves the adrenal glands themselves.
- Secondary adrenal insufficiency involves insufficient ACTH signalling from the pituitary.
- Tertiary adrenal insufficiency involves insufficient signalling from the hypothalamus.
The distinction matters because the clinician is not only asking whether cortisol is low. They may also need to understand which part of the signalling pathway needs further assessment. A CRH result is considered alongside symptoms, medical history and other investigations, rather than used as a stand-alone answer.
CRH testing can also be used in some investigations of excess cortisol. The clinical question and interpretation are different from an assessment for adrenal insufficiency. If you have been offered the test, ask your team what they are looking for in your situation.
Is a CRH test the first test for low cortisol?
Not usually. A clinician will start by considering your symptoms and medical history, then decide which tests are appropriate. The ACTH stimulation test is used more often in the investigation of adrenal insufficiency.
A CRH test is a more specialised investigation, not a routine test for everyone who feels tired. Your GP can assess the wider picture and arrange initial investigations or specialist advice where appropriate.
CRH Test Versus ACTH Stimulation Test
The names are similar, but the tests begin at different points in the hormone pathway.
In a CRH stimulation test, the clinician gives CRH. The test looks at the response to a signal directed at the pituitary. ACTH is measured at intervals, and cortisol may also be measured.
In an ACTH stimulation test, the clinician gives a manufactured form of ACTH. Blood cortisol is measured before and after the injection to see how the adrenal glands respond to that signal.
You may hear the ACTH test called a short Synacthen test in the UK. Although both tests can contribute to an adrenal assessment, they do not answer exactly the same question, so one set of results should not be interpreted using the rules for the other. (leedsth.nhs.uk)
The NIDDK guidance on adrenal insufficiency tests notes that an ACTH stimulation test may be less informative in recently developed secondary adrenal insufficiency. The adrenal glands may still respond to ACTH even when the pituitary signalling problem is recent. This is one reason a specialist considers the timing and clinical history as well as the test result. (niddk.nih.gov)
How is either test different from a single cortisol blood test?
A single cortisol blood test provides a snapshot of cortisol at the time it was taken. A stimulation test looks at a response across several samples after a specific hormone has been given.
That difference matters because cortisol changes during the day, and a result needs to be considered in relation to its collection time and the reason it was requested. A single result may be useful to a clinician, but it cannot reproduce the information from a supervised stimulation test. (ncbi.nlm.nih.gov)
A private cortisol result is not a substitute for a CRH or ACTH stimulation test when a clinician is investigating possible adrenal insufficiency.
What Happens During a CRH Stimulation Test?
Your appointment instructions should explain the procedure used by your clinical team. The details can vary, but the general sequence is:
- A healthcare professional places a small tube called a cannula into a vein, usually in the hand or arm.
- A blood sample is taken before CRH is given. This is the baseline sample.
- CRH is given through the cannula.
- Further blood samples are taken at planned intervals to assess the hormone response.
The time points and the hormones measured depend on the protocol and the question being investigated. The team will tell you how long to allow for the appointment and when you can leave.
If you are worried about needles, fainting during blood tests or having a cannula, tell the team beforehand. They can explain how they will collect the samples and what support is available during the appointment. Tell them promptly if you feel unwell at any point.
How should you prepare?
Follow the instructions from the team performing your test. If the letter does not make something clear, contact them before the appointment rather than relying on preparation advice for a different hormone test.
Useful questions include:
- May I eat and drink before the test?
- Should I take my usual medicines at their normal times?
- How long should I allow for the appointment?
- What happens if I am unwell on the day?
- Who should I contact if I need to rearrange it?
Give the team a complete list of prescribed and non-prescribed medicines and supplements. Include steroid medicines in all forms, such as tablets, inhalers, creams and injections, as well as any recent changes. Also mention hormonal medicines, including contraception or hormone replacement therapy.
Do not stop or change prescribed medication unless the clinician responsible for your care tells you to. This is especially important with steroid medicines. If you are unsure what to do before the test, ask the team for individual instructions.
Understanding ACTH and Cortisol Results
A CRH test produces a series of results, not just one number. The clinician looks at the baseline and subsequent samples, including how ACTH changes over time. If cortisol is measured, its response is considered alongside the ACTH results and the wider clinical picture.
A limited ACTH response may prompt further consideration of pituitary signalling. A slower ACTH rise may point towards a different pattern of signalling. These patterns are clues for the specialist, not conclusions to draw from one result in isolation.
The timing of each sample matters. So do the laboratory method, medicines, the purpose of the test and the rest of your health history. A value that appears outside a printed reference range does not by itself explain where a problem lies or what should happen next.
Likewise, a cortisol threshold quoted online for an ACTH stimulation test should not be applied to a CRH test. The substances given and the questions being asked are different.
Questions to ask about your report
If you receive a copy of your results before speaking to your clinician, it can help to note:
- Which results are ACTH and which are cortisol?
- Which sample was taken before the injection?
- When were the later samples taken?
- What response was the team looking for?
- How do the results fit with my symptoms and earlier tests?
- Do I need further assessment or follow-up?
You do not need to work out the pattern yourself before your appointment. Take the full report, including units and collection times, to the discussion. Ask the clinician who requested the test to explain what it means for you.
Symptoms, GP Review and When to Seek Help
Fatigue, dizziness and feeling generally unwell can have many possible explanations. A cortisol test is not a way to identify the cause of every symptom, and a private result should not delay an assessment.
Make an appointment with your GP if symptoms persist, worsen or are concerning. Explain how they affect daily life and mention any medicines you take or have recently changed. A history of steroid treatment is particularly useful information for the clinician.
While waiting for a routine appointment, a brief record can make it easier to describe what is happening. You could note when symptoms started, whether they occur at a particular time, changes in sleep or energy, and any dizziness, nausea or unintended weight change. Bring your medication list and relevant previous results too.
Tracking is a way to prepare for a conversation, not a reason to wait if you are becoming more unwell.
When is help urgent?
Sudden severe symptoms need urgent attention. If someone collapses, has severe confusion, has difficulty breathing or is seriously unwell, call 999 or go to A&E.
If you have known adrenal insufficiency and become acutely unwell, follow the emergency plan given by your clinical team and seek urgent help. Do not wait for a private test result or a routine appointment.
For worsening symptoms that are not immediately life-threatening, contact your GP promptly or use NHS 111 for advice on where to get help.
Choosing the Next Step
A CRH stimulation test gives a clinical team information about the response to a hormone signal. It can be useful in a specialist investigation, but it is not interchangeable with a single cortisol measurement or an ACTH stimulation test.
At Blue Horizon, we see blood results as a snapshot to support a conversation with a healthcare professional. A broader set of tests is not automatically more useful, particularly when a clinician is already investigating a specific hormone pathway.
Start with your GP for persistent symptoms, keep a practical record of what you have noticed and consider further testing only in the context of the question your clinician is trying to answer. If you have been offered a CRH test, ask the team who arranged it to explain its purpose and preparation instructions.
FAQ
Does a CRH test measure cortisol?
It can. A CRH stimulation test assesses the response after CRH is given, with blood samples taken over time. ACTH is measured, and cortisol may also be measured depending on the protocol. Your clinical team can confirm which results to expect.
Can a CRH test show adrenal insufficiency?
It can provide information that helps a specialist investigate adrenal insufficiency, particularly questions about pituitary and hypothalamic signalling. The result needs to be considered with your symptoms, history and other tests.
Can I take a CRH stimulation test at home?
A CRH stimulation test involves an injection into a vein and timed blood samples under clinical supervision. A home blood sample measuring cortisol does not replicate that procedure.
Should I stop taking steroids before a CRH test?
Do not stop or change steroid medication on your own. Tell the team arranging the test about all current and recent steroid use, including inhalers, creams and injections, and follow their individual instructions.
