Synacthen Test

Authored by , Reviewed by Dr Colin Tidy | Last edited | Meets Patient’s editorial guidelines

A synacthen test checks the function of your 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, often 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 your 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 adrenal glands are endocrine glands. This means they make and release (secrete) hormones. Hormones are chemicals which are released into your 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 your adrenal glands make different chemicals (hormones).

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 your blood pressure by helping to regulate the salt and water balance in your body. Androgens are hormones such as testosterone and these hormones contribute to growth and reproduction in men and women.

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

The release of cortisol is controlled by hormones produced by two parts of your brain - the hypothalamus and the pituitary. The hypothalamus receives input from your 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 your pituitary gland to release (secrete) adrenocorticotropic hormone (ACTH).

ACTH is released into your bloodstream and travels to your adrenal glands. Here ACTH causes cells within the outer part of your adrenal glands (the adrenal cortex) to make more cortisol. High levels of cortisol stop your 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 your body 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 your 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.

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 first thing 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 your 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 sometimes 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 your 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.

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, your vein used to take the blood may become swollen; this usually settles within a few days. 

You cannot have this test if you are pregnant or have had a reaction to synacthen in the past. If you have recently had an operation on your pituitary gland then you will usually have to wait at least six weeks before having this test.

If you are taking the oral contraceptive pill or hormone replacement therapy then you are usually asked to stop this six weeks before you have the test. If you are taking steroid tablets then you should not take them for 24 hours before this test. Your doctor needs to know if you are taking any medicines that contain steroids (for example, inhalers or creams).

Further reading and references

  • Ceccato F, Scaroni C; Central adrenal insufficiency: open issues regarding diagnosis and glucocorticoid treatment. Clin Chem Lab Med. 2019 Jul 2657(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 2399(43):e22621. doi: 10.1097/MD.0000000000022621.