Brief Answer:
Most likely it is due to Prednisone.

Detailed Answer:
Hello!
Welcome to Healthcare Magic.
I read carefully your query and understand your concern.

Your cortisol level is low and the most possible cause considering your medical history ,is Prednisone intake.

Cortisol is a hormone produced by adrenal glands.This production is under the control of a hormone called ACTH produced by pituitary gland in brain.The pituitary produce this hormone in response to the body’s need for cortisol.
This works like a negative feedback circuit.This means that when Cortisol level in blood is low, this induce production of ACTH in pituitary gland that induce adrenal glands to produce more Cortisol.In this way the cortisol level in blood increases.

Prednisone is a Glucocorticoid,which are steroid hormones that act like cortisol.
These medications have an effect on ACTH because the cells in the pituitary recognize any of these drugs, they sense that there is cortisol present and therefore produce less ACTH. This makes the adrenal glands to produce less cortisol.
This low cortisol level due to ACTH suppression from glucocorticoid medication can be very temporary or permanent depending on the dose and length of use of the medication. A few days of prednisone will not produce a significant problem, but several weeks of prednisone at a dose of 10 mg will decrease the cortisol level.
Symptoms related to low cortisol level depend on the rate of reduction of cortisol.The most common symptoms are severe fatigue, loss of appetite, weight loss, nausea, vomiting, diarrhea, muscle weakness, irritability, and depression.


Hope it helps.
Let me know if you have any other question or doubt to clarify.I will be happy to assist you further.

Kind regards,
Dr.Rovena Murati

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Brief Answer:
detailed below.

Detailed Answer:
Hello again!
I apologize for the delay in answering you.

In your case it is very important to consult an Endocrinologist.

Since you are having symptoms of Adrenal Insufficiency and your Cortisol level is low , probably you need to take oral Glucocoricoids until your symptoms are gone. Cortisol is replaced orally with hydrocortisone tablets, a synthetic glucocorticoid, taken once or twice a day.

It may look strange to you that since Glucocorticoids caused the problem ,you have to take them to treat the condition but I will explain it.
As I said in my answer above ,taking Glucocorticoids , decreases the level of ACTH produced by pituitary gland and this makes adrenal glands to produce less cortisol.So during the treatment with glucocorticoids adrenal glands are underactive like sleeping.
This is not a problem as long as the tablets with the synthetic steroids continue to be taken at the correct dose. However, if they are suddenly stopped,underactive adrenal glands can't produce enough cortisol.
Therefore, if synthetic steroids are no longer needed their doses need to be
gradually reduced over several weeks or months to ensure that the adrenal glands wake up again and start producing cortisol.

So your doctor may put you again in Glucocorticoids until cortisol level increases to normal level in blood and your symptoms improve and then he will reduce the dose gradually until your adrenal glands are completely working and produce normal levels of cortisol.

Hope it is easy to understand (it may look a bit complicated).

Let me know if you need further help.
Take care.

Dr.Rovena

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