What is a Cortisol Stimulation Test?

A cortisol stimulation test, sometimes known as a synacthen test, is used to investigate the production of the hormone cortisol. Cortisol is normally released by the adrenal glands, located on top of the kidneys, in response to a hormone known as ACTH, which is produced by the pituitary gland in the brain. During a cortisol stimulation test, a synthetic copy of ACTH is given by injection, and cortisol levels in the blood are measured before and after the injection to see if they rise as expected. If low cortisol levels are found after the test, this could indicate a problem with the adrenal glands.

Cortisol is a necessary hormone which acts throughout the body, helping it react to stress, fight off infection, and maintain blood sugar and blood pressure within normal limits. If the adrenal glands are not functioning properly this can lead to low cortisol levels, which are associated with a serious condition called Addison's disease. In Addison's disease, symptoms affect the whole body but include tiredness, dark patches on the skin, low blood pressure and nausea. The risk is that, without treatment, a stressful experience such as an infection could make symptoms suddenly worse, leading to a collapse which could be fatal. A cortisol stimulation test is useful to help diagnose the disease.

There are short and long versions of the cortisol stimulation test. In the short version, cortisol testing involves having a sample of blood taken from a vein to measure cortisol levels. Next, an injection of synthetic ACTH is given, into a muscle or vein. After waiting for around half an hour, a second blood sample is taken and cortisol levels are measured again to see if the adrenal glands responded normally to the injection. The longer version of the test is similar but, instead of taking just the second blood sample, a whole series of samples are taken to assess cortisol levels and adrenal gland function over 24 hours.

As well as helping to make the diagnosis of Addison's disease, a cortisol stimulation test can indicate whether other diseases are affecting the adrenal glands, such as infections or cancer. If a diagnosis of Addison's disease is made, treatment consists of taking a cortisol replacement two or three times a day. Different doses are used, starting the day with higher morning cortisol levels and ending with lower evening cortisol levels, in order to mimic the pattern found in most healthy people.