Synacthen Test

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A synacthen test checks the function of the adrenal glands. It can help to see whether your body is producing enough steroid hormone (cortisol).

Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.

A synacthen test uses a special chemical to test how well the adrenal glands make a chemical (hormone) called cortisol. It involves stimulating the adrenal glands and then checking to see if they respond. Synacthen is another name for tetracosactide, the chemical used in the test.

The adrenals are endocrine glands. This means they make and release (secrete) hormones. Hormones are chemicals which are released into the bloodstream. They act as messengers, affecting cells and tissues in distant parts of your body. The adrenal glands produce hormones which help to control a number of your body's functions.

You have two adrenal glands which are found just above each kidney. About the size of a walnut, each adrenal gland has an outer part (adrenal cortex) and an inner part (adrenal medulla). Cells in different parts of the adrenal glands make different chemicals (hormones).

Cells in the outer part of the adrenals (the adrenal cortex) make the chemicals (hormones) cortisol, aldosterone and androgens.

  • Cortisol is a steroid hormone and is vital for health. It has many functions which include:
    • Helping to regulate blood pressure.
    • Helping to regulate the immune system.
    • Helping the body to respond to stress.
    • Helping to balance insulin in regulating blood sugar level.

Each adrenal chemical (hormone) is controlled in a different way.

The release of cortisol is controlled by hormones produced by two parts of the brain - the hypothalamus and the pituitary. The hypothalamus receives input from the body about the state of many different bodily functions. When the hypothalamus senses levels of cortisol are low it releases a special hormone, called corticotropin-releasing hormone (CRH). CRH travels to the pituitary via the connecting blood vessels. CRH stimulates the pituitary to release (secrete) adrenocorticotropic hormone (ACTH).

ACTH is released into the bloodstream and travels to the adrenal glands. Here ACTH causes cells within the outer part of the adrenals (the adrenal cortex) to make more cortisol. High levels of cortisol stop the hypothalamus and pituitary from secreting more of their hormones. This system ensures that cortisol should only be made when the level of cortisol is too low.

pituitary and adrenals

During the synacthen test you are given a chemical called tetracosactide (synacthen is the commercial name of this chemical). Tetracosactide is a chemical copy of ACTH.

ACTH is the chemical (hormone) released by the pituitary, which stimulates the adrenals to produce cortisol. If the adrenal glands are working properly they should respond to the tetracosactide by producing cortisol. Levels of cortisol are checked by taking a blood sample.

If levels of cortisol remain low, despite the tetracosactide injection, this suggests there is a problem with the function of the adrenal glands. See also the separate leaflet called Addison's disease.

A synacthen test can be carried out in several different ways. Your local hospital should tell you how the test is to be carried out.

You will usually be asked to come into hospital in the morning. Here a sample of your blood will be taken to see how much cortisol is present before your adrenals are stimulated. You may have a small needle called a cannula put into a vein in the back of the hand. This allows doctors to take blood samples and give the tetracosactide.

You will then be given an injection of tetracosactide either into a vein via the cannula or into a muscle. In the so-called 'short synacthen test' a sample of blood will be taken again approximately 30 minutes after you received the injection of tetracosactide. This is to measure the levels of cortisol in the blood to see if the adrenals have responded to the tetracosactide. If you are having the short test you will then be free to leave.

Less commonly, you may have what is known as a 'long synacthen test'. In this type of test you give a blood sample at the start of the test. You then have an injection of tetracosactide and blood samples are taken at regular intervals over a 24-hour period.

A synacthen test is used to check the function of the adrenal glands. It can also give doctors information about how the hypothalamus, pituitary and adrenal glands work together. The test may also be used to diagnose Addison's disease.

Usually there is very little preparation needed for a synacthen test. Your local hospital should give you more information if you need to do anything specific.

There are usually no side-effects from a synacthen test apart from a small bruise which may appear at the place where the needle was inserted. Rarely, the vein used to take the blood may become swollen; this usually settles within a few days.

Original Author:
Dr Rachel Hoad-Robson
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
12702 (v2)
Last Checked:
Next Review:
The Information Standard - certified member
See also: Adrenal Insufficiency and Addison's Disease written for professionals

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