Thinning eyebrows is an indication of hypothyroidism. While TSH is not the only indicator of thyroid issues, it is one, and yours being at 5 - 6.5 is too high. You should have your actual thyroid hormone levels tested. Those are Free T3 and Free T4.
When you went hyper from over medication, you should have simply lowered your dose, not stopped taking the med. Since you've been diagnosed with Hashimoto's your thyroid levels will continue to get lower and lower (TSH will go higher) because your thyroid function is diminishing; eventually, your thyroid will produce no hormones and you be completely dependent on replacement hormones in order to live, since every cell in the body requires thyroid hormones.
well when I was hyper my sides of my eyebrow were very thin to none existant but now i'm hypo due to RAI and on meds, and they are coming back. Thinning side brows is an indicator for thyroid problems, learned that on Dr. Ozz by a women doctor who looks at the face and can detect health issues. I just thought my thinning eyebrows was because I was getting older. well i hope yours comes back soon.