Phoebe71... Could you please post your most recent thyroid related blood tests? Those should include Free T3, Free T4 and TSH that you should be getting tested for, periodically - at least, annually, if not more often. Please be sure to include reference ranges, as those vary from lab to lab and have to come from your own report.
GERD is a very common symptom of hypothyroidism, so that could mean that your Synthroid dosage isn't high enough or that you aren't converting Free T4 to the usable Free T3.
Iron is necessary for the conversion of Free T4 to Free T3, and low ferritin is an indicator of low iron. Have you had a complete iron panel done? Do you take iron supplement and/or eat plenty of iron rich foods?
The low energy could be a result of, either, too low thyroid hormones, low iron or vitamin B-12 deficiency. If you haven't had B-12 tested, you should get that done, as well.
Low blood sugar can happen for a variety of reasons, but you should eat small meals throughout the day to make sure you keep it steady and don't eat foods with high sugar content, because that spikes blood sugar and insulin leves, then the bottom drops out.
Once we know the answers to the above questions, we can further help...