text: underweight? hi, i'm 13. i'm 4'11 and i weigh 75 lbs. i know i'm underweight. i have a very fast-metablism. my question is how do i slow it down? your ideal weight range is between 98 and 123 lbs. you can check it here http://www.healthstatus.com/calculate/iwc  you are obviously underweight. since you talk about very fast metabolism, do you have problems tolerating hot weather, or sweat a lot or have palpitations? these are all features of overactive thyroid and there are many more.  the second question is have you been well without any health problems because certain illnesses can make you lose weight.  thirdly, there are people who have fear of eating or obsessed with weight loss such as anorexia.  all three options need a chat with a doctor. if all is well, after the check up then a dietitian or nutritionist or the doctor can point you in the right direction