Welcome, Clau123. The symptoms you describe are typically related to hypothyroidism. It is possible that your remaining thyroid gland is no longer producing sufficient hormone for your body's needs.
It is definitely worth trying Synthroid or some other form of thyroid medication. Your doctor can start you on a relatively low dose. Give it at least 4-6 weeks to stabilize (it takes that long for your body to fully assimilate the dosage) and see how you react.
Since you are very familiar with the symptoms of hyperthyroidism, you will quickly recognize if the dosage is too high and can inform your doctor that you need a reduction. The best thing to do is to slowly increase your dose, with blood tests every 2-3 months to see what your hormone levels are doing. If you get to a point where you're starting to have hyper symptoms, your doctor can lower the dose a little and see how that works. Your symptoms will go away when your thyroid hormone is at the correct level. And if the dose is too high for you, it can be easily reduced and relieve the hyper symptoms.
Just be aware that you may experience symptoms of hypo, hyper or a combination of both during the 4-6 weeks following a dosage change.
What caused your original hyperthyroidism? Was it related to nodules or a goiter? Have you had your antibodies checked? There are supplements you can take to keep things balanced.
If you've not already done so, read through our Information Archive and Stories threads. There's a ton of stuff you may find quite helpful there:
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