Genetics, low-fat diets and other factors may all cause a child to be slightly slimmer than his same-age peers. However, when the child's weight falls below the fifth percentile for his age and height, physicians will consider him to be underweight. Your child's pediatrician will run a series of laboratory tests and other physical exams to determine the cause of his weight loss or inability to gain weight.
Some underweight children have digestive conditions such as gastro-esophogeal reflux disease (GERD) or inflammatory bowel disease. In most severe cases, the underweight child displays obvious symptoms such as chronic vomiting or diarrhea. In other cases, the child refuses to eat normal quantities because he experiences heartburn or bowel pain after eating. According to babycenter.com, a pediatrician will investigate chronic vomiting or diarrhea a possible cause of unexplained weight loss in a child.
If both of your child's parents are very slim, it is likely that her genetics play a role in her thinness. A child may be unusually thin because she inherited a small frame from both sides of her family. When a pediatrician suspects that a child's lineage plays a role in her thinness, he may not prescribe a specific treatment. However, experts will generally seek other potential explanations before assuming that a child's genetics alone are responsible for her size.
The American Academy of Pediatrics notes that eating disorders are alarmingly prevalent in children—pediatric eating disorders have become significantly more common in recent decades. These serious mental illnesses, which tend to have their onset during the preteen years, can trigger children to diet excessively, significantly limit their food intake or self-induce vomiting after a meal. A pediatrician may refer an underweight child to a psychologist or psychiatrist to examine for signs of disordered eating.
Hyperthyroidism, a hormonal condition marked by an unusually active thyroid gland, can cause a child to become underweight. According to Mayo Clinic, thyroid diseases can be difficult to diagnose without blood testing because they mimic the symptoms of other conditions. A child with hyperthyroidism may be unusually nervous or irritable, and a physical exam may reveal a rapid heartbeat. If blood testing reveals elevated levels of thyroid hormone, the pediatrician may recommend radiation therapy, surgery or other treatment options.