underactive thyroid
"Nature Throid was a disaster for me.  I was trying to switch from Armour Thyroid (since it was getting pricey), and had been taking NT for only about a week.  They dosage differences were a little tricky, but from the major crash I experienced, you'd think I was taking a placebo.  It was as if my body rejected it outright.  Much as I hate having to take a more expensive medicine, my doctor advised that I go back to Armour, at least for now."