I Had Thyroid Removed In 2001 And Have Been Taking

I had thyroid removed in 2001 and have been taking oral thyroid meddication successfully since. 10 weeks ago at bi-annual physical my TSH level was 58, rather than the .300 to 3.0 that it typically has been over the years and was within that range just 6 months previously. Doctor did a recheck within a few days and it was 38. Changed from generic levothyroxide to Synthroid but after 6 weeks, no change, still at 38. He has now doubled my med from .05 to 1 and going back in 6 weeks to recheck. I started daily magnesium just after my check-up 6 months ago and that was the only change to meds during the period that the TSH level rose to 58. I had started arimidex about 5 months prior to that due to breast cancer, but TSH level had remained good during those 5 months. Do you think magnesium supplenent could be affecting absorption of synthroid, although I take thr magnesium at night and synthroid first thing in morning. Or do you think there is an interaction built up from arimidex, or could this be a pituatory gland issue? Do you know of any other reasons the TSH levels would go so high when I take my medicine every morning, same time, on an empty stomach?