We heard back from the endocrinologist about Isla's test results today. They tested for thyroid problems, growth hormone deficiency, kidney function, anemia, celiac disease, and Turner syndrome (and maybe a few other things that I am forgetting). And all of those results were normal!!
The only abnormal result was in the bone age x-ray. It showed delayed bone age, which is actually a good thing. The x-ray shows that her bone age is younger than her actual age now, which means there is still room for her to grow. At her height, if her bone age was normal or advanced, that would be more cause for concern, as it would limit her ability to grow in the future.
Her official diagnosis from the doctor was "late bloomer" which is also called constitutional growth delay. Kids with constitutional growth delay often fall off the growth curve during the infant and toddler years, because they are not growing at the same rate as other kids. According to what I have read, Isla should start to catch back up to the growth curve in the next few years. Then she may be delayed in growth/development again around puberty, but she should grow into normal adult stature with no other problems.
So this is great news! I am greatly relieved to not have to do any more tests on Isla and to not have to worry any more about why she is so much shorter than everyone else her age. We just have to follow up with the endocrinologist in six months, as they still want to keep an eye on her growth and development for a little while and make sure nothing changes.