I know a woman who began taking Synthroid when she was 16. She's a 50-something grandmother now and doing perfectly fine. Most of the hypothyroid people I know take Synthroid. They're fine, too.
Hypothyroid is hypothyroid whether it's due to Hashimoto's disease, radiation, or thyroidectomy, and the treatment is the same for hypoT regardless of the cause. Just because some people don't do well with synthetic T4 and need T3, it doesn't mean everyone needs T3.