I don't think the metoprolol
, a beta blocker which; reduces the force and rate
of the contraction of the heart
, is causing this. The medicines that you take for hyperthyroidism might cause some exacerbation of the "hard heart beat" symptoms, especially if they are contraindicated with metoprolol, of course. Your doctor should be on top of this and I would let him know about it, especially if it just started, or is getting worse.