Check out: Link to Vitamin D council
Mine was 14 about 4 years ago. It's taken me about 4 years just to get mine up to 50+. My doc wants me to be around 100. I take 5000 a day. According to vitamin D council most people us about 4000 a day. And according to my doctor, if you have any auto-immune problems your body will soak up D.
If you live far enough south, spend 10-20 minutes a day in the sun with as little on as possible. Have your level checked about this time of year march/april. It'll be the lowest and again at the end of summer around sept./oct. to see what it is at the highest level.
I read-up about sunlight for this post. Was surprised to find that 5-10 minutes of sun exposure for just 2-3 times per week can elevate Vitamin D levels. Perhaps your daughter’s problem is absorption-related.. Due to other medications which can interfere.
Grew up in southern Texas....plenty of sun...no vitamin D issues...but now I get precancerous skin lesions removed every 6 months..... GRIN....really!