I have experience with this same issue and unfortunately, my running post-Hashimoto has never been the same. I am a female who started running at 30-years old, went from my first race of a half marathon that took me over 2 hrs, to a sub-1:25 half and sub 3 marathon by 32. I did not know Hashi ran in my family and for all of my relatives the condition emerged during times of stress. I suspect the stress on my body of going from a non-runner to constantly training to improve my race times brought it out in me. Shortly after my last marathon, I could barely manage one 10 min/mi. and over the next year deteriorated until I could hardly make it up a flight of stairs without sitting down and resting, constantly was suffering from random infections, gained a ton of weight, and was horribly dizzy and lightheaded all of the time. I actually ended up collapsing and in the hospital, with low BP and electrolyte imbalances, and that is where I was finally diagnosed.I started Synthroid and for the first two years felt well enough to live fairly normally (could get up a flight of stairs, hair stopped falling out etc.), but my running was still horrible. I had swings in my hormone levels and needed several medication adjustments, trying different combinations etc. Finally, over the past year or so, my hormone levels have been stable and my running has felt good again. With that said, I still cannot handle the workouts I used to do in order to get back to where I was. I break down much more quickly. Even with the medication, I now gain weight more easily than prior, and have a hard time getting enough nutrition to support training, without piling on the lbs. I just do not have the ability to recover as I once did. I do not think the Sythroid harms my running and it has worked well enough to allow me to lead a normal life. However, the disease process appears to still have some negative effects, even with the medication. Since I was only a few years into running, at the time, I thought I had a lot of room to improve. Now I have accepted I'll probably never even get back there. It sucks, but it is what it is an I have accepted my new reality. I am obviously only one example and so perhaps my experience is not the norm, but after several years of trying out different things without success, I finally decided it was time to accept my body's limitations and just enjoy what I am able. I spend a lot more time on the trails now, as running slower on the roads is pretty depressing and at least I can enjoy the scenery, being in beautiful places and the company of other friends who equally enjoy the outdoors.I hope your experience ends better than mine and that you are able to feel better while running very soon.