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Synacthen test

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

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

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What is a synacthen test?

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.

How does a synacthen test work?

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

ACTH is the chemical (hormone) released by the pituitary gland which stimulates the adrenal glands 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 the separate leaflet called Addison's Disease for more details.

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What happens during a synacthen test?

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

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

An injection of tetracosactide will then be given, either into a vein via the cannula or sometimes into a muscle.

In the so-called 'short synacthen test' (which is the usual test), a sample of blood will be taken again approximately 30 minutes after receiving 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.

Less commonly, a 'long synacthen test' is used. In this type of test, a blood sample is given at the start of the test. After the injection of tetracosactide, blood samples are taken at regular intervals over a 24-hour period.

What is a synacthen test used for?

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

A one-off measurement of blood cortisol is not accurate enough to diagnose Addison's disease as it may be low from time to time in normal people.

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What are the adrenal glands?

The adrenal glands 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 the body. The adrenal glands produce hormones which help to control a number of the body's functions.

Where are the adrenal glands found?

There are two adrenal glands, one of which is 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).

What do the adrenal glands do?

Cells in the outer part of the adrenal glands (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 (glucose) level.

  • Aldosterone is a hormone which also helps to regulate blood pressure by helping to regulate the salt and water balance in the body.

  • Androgens are hormones such as testosterone. These hormones contribute to growth and reproduction in men and women.

How do the adrenal glands work?

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 gland via the connecting blood vessels. CRH stimulates the pituitary gland 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 adrenal glands (the adrenal cortex) to make more cortisol.

High levels of cortisol stop the hypothalamus and pituitary gland from secreting more of their hormones. This system ensures that cortisol should only be made when the level of cortisol in the body is too low.

Pituitary and adrenal glands

pituitary and adrenals

What should I do to prepare for a synacthen test?

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.

Are there any side-effects or complications from a synacthen test?

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.

Who cannot have a synacthen test?

This test cannot be used in someone who is pregnant or has had a reaction to synacthen in the past. It is usual to have to wait at least six weeks after having an operation on the pituitary gland before having this test.

Someone taking the oral contraceptive pill or hormone replacement therapy is usually asked to stop this six weeks before having the test. Steroid tablets should not usually be taken for 24 hours before this test. Topical steroids (via inhalers, nasal sprays or creams) should be discussed with the doctor requesting the synacthen test.

Further reading and references

  • Ceccato F, Scaroni C; Central adrenal insufficiency: open issues regarding diagnosis and glucocorticoid treatment. Clin Chem Lab Med. 2019 Jul 26;57(8):1125-1135. doi: 10.1515/cclm-2018-0824.
  • Butt MI, Alzuhayri N, Amer L, et al; Comparing the utility of 30- and 60-minute cortisol levels after the standard short synacthen test to determine adrenal insufficiency: A retrospective cross-sectional study. Medicine (Baltimore). 2020 Oct 23;99(43):e22621. doi: 10.1097/MD.0000000000022621.

Article history

The information on this page is written and peer reviewed by qualified clinicians.

  • Next review due: 26 Jul 2028
  • 28 Jul 2023 | Latest version

    Last updated by

    Dr Pippa Vincent, MRCGP

    Peer reviewed by

    Dr Toni Hazell
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