I was just reading about this yesterday.......how thyroid replacement can cause cortisol to drop too low. It was in an interview between Mercola & John Lowe (at least I think that's where I read it). I'm doing an adrenal cortisol saliva test today, so hopefully that will give me a clue when I get the results back.I just finished an experimental course of NDT that lasted 4 months. By the end of this period I felt terrible, so fatigued I could barely function. I thought I had developed CFS.
My naturopath suggested testing adrenals with a DUTCH panel test. Interestingly, the results indicated that elevated thyroid levels caused my body to metabolize cortisol at a faster rate which did not leave enough for my body to use. Once I stopped the NDT and started supporting my adrenals I felt immensely better. If your adrenals aren't working optimally you'll have trouble finding success with NDT in my opinion.