Why do I need a synacthen Test?

The synacthen test, or synthetic ACTH stimulation test, is used to assess whether the adrenal glands would be able to respond to a stimulus and produce cortisol, the body's natural corticosteroid. During long term treatment with artificial corticosteroids at a dose above about 8mg prednisolone/prednisone the production of cortisol is suppressed because the complex feedback set-up in the body that governs the functioning of the HPA axis (hypothalamus, pituitary and adrenal glands) gets signals telling it there is already adequate corticosteroid present to allow the body to function properly and no more is required as excess corticosteroid, as we know, causes problems. Once the level of corticosteroid falls below that level, the adrenal glands must come out of hibernation and top up the oral dose to the level required for the body to function. This dose varies from person to person because some people absorb more of their oral dose than others, the bioavailability. In most people, when they reduce slowly, the adrenals recover and produce cortisol again. It is said to take at least a year after ceasing to take any pred before this function is reliable.

However, in some people the adrenal glands do not wake up and so as the dose is reduced, the body is not topping up the requirement and slowly the person enters a state of secondary Addison's disease. In the event of a sudden excessive stress, the body is unable to produce the top-up and, where required, the excess spike of cortisol to allow the body to cope under stress. This may trigger an Addisonian crisis.

It has generally been thought that, providing patients reduce their dose of pred slowly enough, the adrenal glands will come out of hibernation and produce cortisol. In fact, it is becoming apparent that more people than it is thought DON'T start to produce endogenous cortisol while reducing their pred dose. Down to about 2 or 3mg pred they may still be functioning well and not notice any problems although others may experience overwhelming fatigue as a sign that all is not well. One lady on the forums was at 2mg pred and felt fine. It just so happened her rheumy asked for a synacthen test and it was found she has no endogenous adrenal function at all. She is at an increased risk of an Addisonian crisis and could become seriously ill. The synacthen test showed her adrenal glands would not function. That is all that test shows really - that they cannot produce cortisol under their own steam, and that the patient requires a replacement therapy, usually with hydrocortisone but also sometimes with pred. In that case, the patient needs to be referred to an endocrinologist for expert assessment.

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