I have a 4 1/2 year old that needs to gain weight. He is perfectly healthy (other than being underweight).?

Does she eat a lot and not gain weight? Does she not want to eat? If she eats fine and she isn't gaining any weight I would have her tested for hypoglycemia. My son has always eaten very well but never gained weight. Then one day when he was three he ped out on my living room floor right in front of me, it was because he was hypoglycemic and he was having a spell. Because he is hypoglycemic his matabolism is very fast so that makes it very hard for him to gain weight even though he eats just like a normal kid, maybe even more I think. Just a thought.