Thank you for visiting the family practice forum.
The normal range for cortisol levels varies, depending on the time of day that they are being taken. For example, at 8AM a normal level is 5.0-23.0 ug/dl while at 4PM a normal level is 3.0-15.0ug/dl. Therefore, the time of day when the level is drawn is definitely a factor to consider here. Now, what causes a low cortisol level? Well, problems with the adrenal glands such as Addison's Disease come to mind (others include congenital adrenal hyperplasia and pituitary insufficiency). This is commonly associated with a low serum sodium and high serum potassium.
Have you had your thyroid function tested? Hypothyroidism is a very common cause of fatigue in women but is also seen in middle aged men fairly commonly. You may consider having thyroid functio testing as well as further testing of your adrenal gland function (these are blood tests) before trying any medication.
I have friends and colleagues who are board certified urologists. The topic of "low testosterone" comes up quite commonly and it is their opinion that too much is made of these results. If you really want to be sure that you have no abnormality of these "hormones", further blood tests to determine the functioning of the pituitary gland and adrenal gland should be considered.
I suspect an endocrinologist would be the best specialist for you to see to have these tests performed.