Although I have never had a positive test for covid, I do share some similarities with you.
Many years ago I was diagnosed with post viral fatigue syndrome, and I lived with that for 4 1\2 years before regaining a better form of health.
I was diagnosed about 25 years ago with an underactive thyroid. Last December my thyroid had dropped enough that I had to increase my thyroixine to 125. Although I gained a little more energy, that soon disappeared.
I am on estradot 100 and mirena coil.
My energy levels vary greatly throughout the months. Today I was fairly active, a walk and some gardening, a rare day of exercise, two weeks ago my joint pain, muscle fatigue and energies so low my husband had to help me out of bed and I struggled to walk more than 20 mins.
Until last year I was doing two long fast walks every day, and a full and very active long day. This has gone. Most days I can only achieve a fraction of what needs to be done each day, let alone what I actually want to do. And my muscles, energy and joint pain is affected by any form of exercise.
It is this crushing fatigue that has led me to speak with my g.p in a couple of weeks. I am aware from other threads on here that there is a big link between thyroid issues and hrt and the way they interact. Also just for the fun of it all peri symptoms seem affected too. Oh good......

Until I speak to my g.p and hopefully get bloods done, I am reluctant to look too closely into causes of this intermittent yet debilitating crash of exhaustion I get, in case my foggy brain gets overloaded with too much info which may or may not be relevant, or I convince myself of a condition or illness based on matching symptoms! Health anxiety can rear up maybe....As you say QoL is so bad.
Some of my peri symptoms are improving, thanks to the hrt, like night sweats, anxiety, anger, VA ongoing treatment. BUT the exhaustion and joint pain affects my life too much to have a stable quality of life.
I just wanted to say I do understand how you feel. I am not anywhere near finding out what the hell is going on with my conflicting health difficulties but it does sound a bit similar to your own.
Am not sure this helps you much, but I think there must be others out there working with these particular issues.
Wishing you well
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