There are many causes of fatigue: -depression -anxiety -medications -viruses such as mono and other infections -anemia -thyroid and other hormonal imbalances -tumors/cancers -obstructive sleep apnea Therefore, it could be multifactorial, and in some cases, unexplainable. Many patients with chronic fatigue recall significant stressors (e.g., major medical procedure, loss of a loved one, loss of employment) in months before symptoms began. In your case with low cortisol, you can take all the vitamin, supplements, and calories you wants, but you wont feel better until you get your cortisol and Addison s disease under-control. I would not do another Hcg diet until there is clear evidence that the Hcg has no effect on cortisol levels and/or fatigue. If the cortisol does not help, then you need to see a psychiatrist for cognitive behavioral therapy and a physical therapist for graded exercise therapy.