I'm no expert, but after having delved into this subject quite often, my best guess is that something might be wrong with your LIVER. The Liver is 1 of the most important Detoxifiyers . It sounds like an overload on it… too much of some toxin either acute or long term build-up .. (like the straw that broke the camels back).
over 50 yrs. ago, I got a blood disease from Bacteria I had picked-up in the Ocean in the Virgin Islands.. It caused INTENSE Itching – especially on palms, arches, back… and all over allergic reaction after a hot shower….long story short- months later I would up w/ Staffoccocal pneumonia…. anyway, just something to ask your doctor about.
@kwm had bloodwork done to test liver and thyroid. Both in normal range, but still itching.