Selenium wouldn't make things worse. At best, it would reduce TPO antibodies somewhat and help with the conversion of T4 to T3. But it would not increase TSH or reduce T4/T3.
I'm not really sure exactly what causes the condition to worsen over time. It probably has to do with the gland continuing to fail even after we begin treatment years to decades earlier. But don't quote me on that.
Sometimes, things just aren't so easy to explain.
Welcome to the board.
Hope we can continue to be of help to you.
Good luck with the Armour!