doesnt t3 bypass the feedback system?

[Mr.Dank]

I read post about people worried about doing damage to their thyroid, But I thought I read that T4 is produced then sensed by I think the hypothalamus which regulates the thyroid via thyroid stimulating hormone. And the thyroid converts the t4 into t3, so t3 is out of the loop. Is this correct? Is there some sort of feedback for t3? Maybe on t4 to t3 conversion?
I think the real danger of t3 is heart damage right?