Dear doctor, Why am I so skinny, but can t put on any weight? I m a 14 year old boy and am too skinny. I used to have a really outgoing, friendly personality, but I ve become more and more self-concious about my weight and height. I m roughly 5ft10 and weigh only 7.5 stone . I do eat well, but I just can t seem to gain any weight. This has made me think I might have some sort of disease (like Marfan s syndrome or something). I can wrap my whole hand around my wrist , with my pinky ( little finger ) overlapping the nail of my thumb . I thought I probably just had an ectomorph body type, but I haven t seem anybody with wrists and ankles as thin as mine. My brother and mother have quite similar characteristics as mine (which makes me think that it might be some hereditary condition), but I seem to have it on a slightly more extreme level. Any help would be much appreciated. Thank you.