text: I am the EXACT same way!\n\nI have been 100 lbs for so long...doctors have ran thyroid tests and other tests also.\n\nHave you had your doctor test you for anything that could be the cause?\n\nI was just diagnosed a year ago with lymphocytic colitis - my body has a hard time retaining nutrient and water.  My doctor now thinks this may be a reason WHY I can't gain weight.  If you have something wrong with your gastrointestinal tract, it may be a cause.\n\nMy doctors haven't really ever told me a way to gain weight - the only thing they've said is to go on a "high calorie diet" - basically just eat, eat, eat (healthy stuff of course, not junk food completely).\n\nI've gained weight doing that...not a whole lot and it takes a LONG time - so have patience; it might not happen in even a year.\n\nGood luck!