What is a LDDST?
The LDDST is a test to see if you are producing too much of a hormone called cortisol. Cortisol is a steroid hormone which is produced by the adrenal glands and is vital for health. It has many functions which include:
The amount of cortisol that is made is controlled by another hormone called adreno-cortico-trophic hormone (ACTH for short). ACTH is made in the pituitary gland (a small gland that lies just under the brain). ACTH passes into the bloodstream, is carried to the adrenal glands, and 'stimulates' the adrenal glands to make cortisol.
The LDDST works by giving you a short course of a steroid called dexamethasone. If everything is working normally this should cause your body to suppress the production of its own steroid hormone cortisol. If your levels of cortisol do not suppress after dexamethasone it may mean that your body is producing too much cortisol of its own (a disorder called Cushing’s syndrome).
How should I prepare for the test?
You need to stop any oral oestrogen therapy or implants 6 weeks prior to test as this can affect the test results. This includes the oral contraceptive pill (in which case you must take alternative precautions to avoid pregnancy) or HRT. You should not be taking any other steroid medications including tablets, inhalers and creams – if you are unable to stop these you must discuss this with the endocrine team in advance. If you are on tablets for epilepsy please alert the endocrine team in advance. You do not need to fast before the test and you will be an inpatient throughout the test (usually Monday to Thursday unless you need other tests on the same admission).
What will happen during the test?
You should come to the ward on the date of the test prepared for a 3-night stay. A cannula (a small needle with a tube attached for taking blood samples) will be placed in a vein in your arm. You will have some blood samples taken from the cannula on the first day. From the second day, you will begin taking dexamethasone tablets at strict 6 hour intervals (i.e. 0900h, 1500h, 2100h and 0300h) for 48 hours. Further blood samples will be taken from the cannula during the test. Once the test is completed, the cannula will be removed and after you have had something to eat you will be able to go home.
What happens after the test?
When the results of the blood tests are available they will be reviewed by the endocrinology team. You will be notified by letter and your GP will also be informed of the results. If there are any changes to be made to your treatment or further tests that need to be organized, these will be outlined in the letter or someone from the department may contact you by telephone.
What if I have any questions?
You will see a doctor on the day of your test who should be able to answer your queries. If you have any questions before or after your test please leave a message and contact number with the endocrine department at the Hospital where your test is booked. Someone will call you back to discuss things.