I just read that beta-blockers will increase T4 to rt3 conversion. I had no idea, but the fact is that since I was put on beta-blockers three years ago (when I was on long-term sick leave for burnout and depression, with a resting pulse of 120 bpm and a BP of 170/120), NDT started to work less well. I had previously been on 5-6 grains with good results, but now I seem to require at least 8 grains a day. I would never have made the connection between beta-blockers and high rT3 levels, but it will be interesting to see what happens when I wean off the beta-blockers...my doctor and I agreed I will do it this summer when I'm off work and subject to less stress.
There is so much to learn about how things interact, both food, supplements, and drugs...!