Cortisol day curve, or circadian rhythm

Investigation Protocol


Assessment of the level of cortisol excess in a patient with proven Cushing’s syndrome. Assessment of the biochemical control of Cushing’s syndrome in patients on medical treatment.

Preparation and precautions

The patient may eat and drink normally prior to the test. The patient can take their usual medication before attending, but are asked to bring in all their usual medication for this to be documented. Estrogen containing medications, including the contraceptive pill and hormone replacement therapy, should be stopped for six weeks prior to measuring serum cortisol. This is because estrogen induces cortisol binding globulin and leads to elevations in measured serum cortisol. Any steroid containing medications should also be avoided, as they may interfere with the hypothalamo-pituitary-adrenal axis, or cross react with the cortisol assay.


Cannulation may be offered. Insert cannula gauge 20 with a three-way tap, and using the vacutainer connector system or a syringe fill one plain clotted tube for each sample taken. Clearly mark with the patient’s details, date and time of sample.

Samples for serum cortisol are taken at 9am, then at 11am, 1pm, 3pm and 5pm. Where requested, and in patients admitted to hospital, additional samples will also be taken at 9pm and midnight. Clearly mark the patient’s details, date and time on each bottle.

When a blood sample is taken at midnight, it must be clearly documented in the notes, whether the patient was asleep or awake when the practitioner first entered the room. Clearly, most patients will be woken by having a blood test, but assuming the blood is drawn within 15 minutes of them waking, this level should still be undetectably low and so must be documented as 'asleep'. If the patient is already awake, the level is not expected to be low, and so must be clearly marked 'awake'.

Additional blood should be drawn into an EDTA tube for ACTH with the first sample where requested. This sample will need to be labelled, then centrifuged, separated and frozen before transport to the laboratory.

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Patient information sheet

Cortisol day curve or circadian rhythm

Your doctor has referred you to have a series of blood tests performed over a whole day. This is to assess the level of steroid hormones your body is producing. You may take all your usual medication before attending and during this test, but please bring them with you to be documented in your notes.

We will be taking a sample of blood at 9am, 11am, 1pm, 3pm and 5pm. To allow the blood sampling to take place with the minimum of discomfort and stress for you, we usually insert a small tube (a cannula) into a vein in your arm at the start of the test. This will be discussed with you when you arrive.

You may take your other medication as normal on the day of the test. However, if you take the contraceptive pill, hormone replacement therapy, or steroid tablets, nasal drops, inhalers, skin cream, or have recently had a joint injection, it is important that you inform us before the test date. You can contact us either by writing to the address at the top, or by phoning the number given at the end of this sheet.

You will be resting in a reclining chair and you may eat and drink as normal on the day of the test. If you are in the unit over the lunch period you will be offered something to eat. You do not have to stay in the unit for the whole day but you may do so if you choose.You may wish to bring a book or magazine to read. We do have a radio, or you can access Patientline; alternatively you may like to bring your own personal radio/cd player (please bring your own headphones). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged, as this can be disruptive to other patients. However, if this poses a particular problem for you please discuss this with the senior nurse in the unit. 

You should be able to undertake normal activities after the test has finished and it should not interfere with your ability to drive home. 

If you have any queries about the test, please contact: 

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