A few things to consider - I have first hand experience with Lyme's Disease, Wilson's Syndrome / Inactive thyroid and my wife's parathyroid problem.
I had similar symptoms for years. Turns out I had both Lyme's disease, which I killed with Colloidal Silver and Wilson's Syndrome which I am working on with Armour Thyroid. I had an inactive thyroid and was taking Synthroid. But I noticed my exhaustion correlated with low body temperature. In other words, When I felt bad I gave myself a rating of 1-10 and then took my body temperature. My body temp was always very low when I felt bad (into the 95s) and when I felt good it was relatively high in the low 97s). I researched and found that synthroid addresses only T3 and not T4 so I got a prescription of Armour Thyroid which addresses both. Back in the day before synthroid and TSH tests, they used to prescribe (well tolerated) Armour Thyroid by symptoms instead of blood tests. Then along came synthroid and blood tests. Well I have been taking Armour Thyroid for about 3 weeks and my body temperature is improving and my brain fogginess is gone. I am sleeping better and have a bit more energy. Prior to this I had Lyme's disease, damn near a 1, 2 knockout punch. (Lyme's Disease post - //www.curezone.org/forums/fm.asp?i=2217051#i)
ALSO my wife had similar symptoms but had a parathyroid problem which was indicated by high calcium in the blood and low vitamin D. People successfully treated for parathyroid dysfunction describe that it is like getting their life back.