Hi, A..
Unfortunately, I don't have any advice about the lab tests that differs from what you have already received. A pediatric specialist is definitely your best choice.
That said, I did go through the failure to thrive diagnosis with my daughter around the same age. At birth she was 7 lbs, 13 oz, but she fell off of her growth curve. At 12 months, she weighed about 12 pounds. She is the only child I have ever known to "wear out" her baby clothes! Although she fell off the curve, she was in perfect proportion--just small. Today, at 15 years old, she is still tiny. She is just 5 feet, wears a size 4.5 or 5 shoe and a zero in junior clothes. She can even fit into her little sister's size 10 jeans! She is healthy and athletic--just tiny.
Definitely follow through on having your daughter evaluated, I did. Hopefully, like me, you will find that all is well.
Good luck and keep us posted.