My daughter has been diagnosed with celiac disease and is extremely underweight. should i have her blood tested for vitamin deficiency?

See a nutritionist: While blood testing for nutrient deficiency is important, routine testing doesn't cover what she might need. See a good nutritionist who works with functional medicine, or one who is familiar with the specific carbohydrate diet (scd). This diet was developed for people with celiac disease. Look at the book, breaking the vicious cycle, by elaine gottschall, which describes the diet.

Possibly: It is not uncommon for patients with newly diagnosed celiac to be underweight. Once the strict gluten free diet is begun the weight will begin to improve. It is within reason for the doctor to check labs such as a CBC to check for anemia and a complete metabolic panel to assess nutritional status.