I can't speak as to getting cold sores (sorry) but I do know Armour is the only thyroid medication that has even remotely worked for me and I have Hashi's.
After a decade of using every replacement on the market only to wind up in the same out of control situation, I ended up in an ER on a holiday with an acute onset of strep and a huge, swollen thyroid. A scurry of Docs were called in to give their opinion, including an endocrinologist. I was informed that it was either the worst runaway thyroid she'd seen in a long time...or it was cancer. I was sent on my way with antibiotics and told to stop taking the levothyroid and return in a week to her office. The first thing she wanted to know was the history of course. I told her my GP kept insisting he could manage my thyroid and would not hear of sending me to an endo (gotta love HMO's). I gave her a list of all the different brands and doses of T4's we had tried and when asked, told her, no, he had never tested my T3 to my knowledge. A biopsy was done to confirm and rule out. Hashi's yes, cancer, no.
Long story short; turned out to be a run away thyroid because I have a T3 conversion disorder. No T4 alone was going to help. Duh! Armour was prescribed and voila! I responded fairly quickly until the good old HMO insisted on generic forms which set me back right to the beginning repeatedly. I had to go back to Armour and only Armour. For a while it was off the market and of course...another 2 year set back. T3-T4 separately didn't work either. What a pain. Literally! Thought I was just plain out of luck and would end up exhausted and miserable with a head full of fog 24/7. All that could be done was to continue trying different medication ideas. Nothing but nothing was working.
Happy ending; Armour came back on the market.. The Endo asked if I wanted to try it again about 18 months ago. No hesitation from this lady! Had to order it special but it was worth the few days wait. The new formula works better than the original did, keeping me pretty stable most of the time these days.