You need to get antibody testing to see if you have the autoimmune disease, Hashimoto's thyroiditis. If you do, you will be on medication for life.
The antibody tests you need to get are Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TGab). You need them both, because some of us have only one of these, so if they aren't both tested, the condition could be missed. Others have both.
Yes, you can stop obsessing, but if there are other questions we can answer, please don't hesitate to post again.
Oh, ok. I need to stop obsessing!
Usually, thyroid medication is taken for life, but that can also depend on the cause of your hypothyroidism. If it's caused by Hashimoto's, then yes, for sure it will be for life, because the antibodies continually destroy thyroid tissue so production of hormones declines as healthy tissue declines. This requires periodic adjustments to replacement medication doses.
Your thyroid will produce what it can. whether you have medication or not. If, between the thyroid and the medication, you get too much hormones, you will go hyper and have to cut back on the medication.