High heart rate from thyroid medication can be an intolerance showing up -- you can't get the thyroid hormone into your cells, and when it stays high in serum, it can be quite irritating. It affects people in different ways, and high heart rate is yours.
Your iron numbers are a problem. It's possible you've never taken enough iron at once to get over the top with what you need. You need enough iron to use thyroid hormone, and you need thyroid hormone to support good iron levels.
You might want to consider working with a thyroid group to learn about that.
In the meantime, here are a few posts (with threads) about iron and Iodine --