failure to thrive

You really don't have enough information, do you? There could be genetic factors causing her small stature; some children of certain ethnic backgrounds (Middle Eastern, East Indian and Southeast Asian for example) appear to be failing to thrive but are genetically predisposed to be small people. There may be endocrine factors (hyperthyroidism), metabolic factors (celiac, cystic fibrosis) and even cardiac or hepatic defects that could be contributing, hence the tests that were ordered. Something is causing a child with a healthy appetite to be much smaller than expected. But you don't have that information so maybe you're going to have to go with a "soft" diagnosis like delayed growth and development related to small stature of unknown etiology. (Just an observation: chiuldren who are very small for thier age tend to be treated like their size and not their age, so they may be immature.)

Good point, saltwater. You've reminded me of a situation I observed more than 20 years ago, not that there was abuse of the twin but definitely failure-to-thrive. The baby, "S" was the child of a friend; "J", his twin was born with short-gut and spent his entire life in the NICU. When "J" died, I looked after "S" and their older sister while Mom and Dad were busy with all the minutiae involved. "S" was very small, much smaller than I would have thought. (I wasn't yet a nurse, just a mom.) He was being fed a non-milk formula that was very expensive and his mom had been buying the powder to save money. I fed him several times while the kids were with me and he slept or played between feeds. The next day I got a call from the mom asking me how much powder I'd been putting in each bottle... I'd followed the recipe on the can, one scoop per 30mL of water. She told me that was wrong, that the can said one scoop per 3 OUNCES of water. Well, my jaw dropped. Suddenly all became clear. I asked her to read me the label, and then explained how to convert mLs to ounces. She was very upset to learn she'd been starving her baby and after a visit to the pediatrician, little "S" started growing. He's 23 and married now.