Abnormalities in hemoglobin can give you false results. One that would give a false high result in certain assays is the presence of fetal hemoglobin. If this were a concern, you doctor could order a blood test called a hemoglobin electrophoresis. If your daughter's previous hemoglobin A1c readings corresponded well with home monitoring, and you are using the same lab, then it is more likely that your daughter has had higher blood sugars recently. You mention she is taking medications to treat hyperthyroidism.
Hyperthyroidism can make diabetes very difficult to control.
As the hyperthyroidism comes under control, the blood sugars may come down. It is not that uncommon for a teen's home monitoring not to correspond well with the A1c. I attribute it to how difficult diabetes care is to do day in and day out. I would mention to your daughter about the discrepancy, and most responsible teens will get things back in shape. Maybe she just hadn't checked all the readings.