What are your current
thyroid hormone levels? You should be getting tested before each med increase, for Free T3, Free T4 and TSH. Please post the results of your latest tests, along with reference ranges, with vary lab to lab so we can better assess you situation.
Magnesium and other supplements can be taken, as long as they are taken, at least, 4 hours away from thyroid medication.
Once we see what your actual levels are, we'll have a better idea of what's going on.