It's a little late but I just found this. I struggle with this a lot because I love being active. Is there anything one can do to fix/manage this? My ACTH and morning cortisol levels are normal.
I had my total thyroidectomy in August 2016. For January 1 I started my New Year resolutions to eat better (not really low calorie, just better nutrition) and light cardio with some resistance training. I have a whole lifetime of experience of exercise and my body within two weeks reacted in a way it never has before. My muscles were not sore, I got headaches, stomachaches, spotted for over a week, I started getting panic attacks again like I did before surgery (when I was hypo).
I am currently in the process of figuring it out. I believe the presence of a thyroid (whether it is fully functional or not), adapts to changes like differences in activity and calories.
At current, my hypothesis is that my T3 is low and needs supplementation with T3 (not more T4, as my current dose has me exactly in the right range and I am not that far off from the level that made me hyper). I am working with a doctor to rule out some other things; before I can get T3 she is worried about getting my heart rate and blood pressure down (which ironically, from reading I've been doing can both be elevated by low thyroid hormone). Anyway, I stopped exercising and the heart rate and BP are coming down.
I had to quit caffeine, which I hope is temporary.
I had my Ferritin tested, it is 13 with a range of 10-291. I believe ferritin is related to the thyroid hormone, I have seen reports of supplemental T3 raising ferritin but am still researching.
I am in the process of getting noradrenaline tested, which I would expect to be elevated if my body is stressed from inadequate thyroid hormone.
I am also going to get cortisol tested, which is needed to help metabolize thyroid hormone.
Selenium also helps with T4 to T3 conversion.
All of this information is very hard to find, especially that results of studies I've found don't seem to include people who have lost their thyroid altogether. But every piece of information I have, including my own labs, makes me believe I am not converting really well on my own (or am low on T3 for another reason) and I require additional T3. What I might feel like if I were able to start taking it, I don't know, but I hope ruling out all of this adrenal stuff allows me that opportunity.
I have a whole binder full of articles and other stuff printed out with information on this specifically. When I have the opportunity, I am glad to start posting some of the links in case it might help.