Its a problem in your body where you cannot properly convert t4 (thyroid hormone) into the t3 hormone (that goes in to your cells) it causes ALOT of problems. That response was in reference to the guy above. It will not be picked up on ANY blood tests, because the conversion happens "behind the scenes" at a cellular level. It causes some freaky stuff. It happen to me after a massive weight loss. (50 pounds, 40 of them in 2 1/2 months) I'll never go on a crash diet again! It causes: Depression, aniexty, feeling like your going to faint, memory loss, cold hands/feet just overall crapy feeling. Look it up on the internet: wilsonstemperaturesyndrome.com