If you are causing the high blood pressure by taking too much T3 (in the nature throid) that will cause your heart rate to increase and you will also be increasing the force of each contraction it makes.
Magnesium is necessary to enable the processes involved in relaxing muscle (including the smooth muscle of blood vessels), so it can help to reduce blood pressure and heart rate, but if the reason for the increased pressure and rate are due to an excess of T3, you will be solving a problem you are creating instead of just stopping the problem, if you see what I mean. Too much T3 for too long can damage your heart permanently, so please do work with your doc to make sure you are taking the proper amount (post your test results on the forum if you like, and we will help you understand them) - if you still would like to improve your blood pressure thereafter, magnesium is a good idea (and people seem so chronically deficient for some reason I don't understand), but I just want to impress upon you the importance of ensuring your thyroid meds are properly regulated.
