A fasting blood sugar of 91 is within normal range. Have you had a Hba1c test done? This test gives you an average 3 month blood sugar result and would be a better test to watch blood sugar with. A truly normal hba1c is 5.0 or less. If your test came back at this number perhaps you have no need to be concerned.
I assume you are treating your hashimoto with thyroid meds, appropriately.
Have you checked your HbA1c level?