After more than three decades managing CFS with some success, I went into a decline about a year ago.
With new (and more user-friendly) insurance via my wife's job in hand, I started pushing to see if there were any areas of my health that might be contributing to the rebound of fatigue. But all the tests and bloodwork came back "normal."
I accepted normal initially, but then started sifting through the tests (while battling brain-fog).
The first test that made me sit up was free testosterone. At 306 ng/dL I was 6 points over my health groups cut off for TRT despite the best medical evidence standing in opposition to such arbitrary cutoffs. I assembled medical studies, consulted by phone with a leading doctor in the field, got thrown out of the Chief of Endocrinology's office, but finally found a brave doctor who was willing to buck his boss.
For a year I've been injecting testosterone. First every-other week in office visits, then self-injecting once-a-week (the latter being much better as for me it eliminates highs and lows).
I can't say testosterone cured my CFS. It did not. But it helped cut fatigue, brain-fog, and promoted well-being and fitness. Life altering for me.
As I cleared mentally I saw my thyroid numbers were not optimal. Treating. Vitamin D was sub-normal. Treating. B12? Same.
Also discovered I have sleep apnea. Treating.
Together these therapies are helping a great deal. I'd rank the testosterone at the top of the list.
The experience has made be very aware that "normal" and "optimal" are very different measures.