15 yrs with hypothyroidism taking 200mcg test 3 days ago came back 8.0 reduce to 125mcg reason of test ER with atrial fibrillation this the reason?

Incomplete. Your question is incomplete. We don't know what test came back 8.0, what drug you're taking, etc. Let's assume you're asking if the er reduced your levothyroxine to 125mcg with a TSH of 8.0 for atrial fibrillation. That would make some sense from a fib perspective, but the TSH of 8.0 would mean your body needs more levothyroxine, not less, so the picture still isn't clear from your question.
Thyroid replacement. I'm not sure i fully understand your question ( i understand you have space constraints). Starting full dose thyroid replacement can cause atrial fibrillation. If starting replacement in the future, he should start at 25 mcg and raise the dose every several weeks by 25 mcgs until he reaches the full dose.