I have heard there are instances where being post viral can affect thyroid hormones. I don't know too much about it but heard recently some mention it happened to a family member after having covid.
If the effects were substantial on a population level basis, I would think one of the countless post-Covid studies looking at this would have picked up a clear signal? It's very possible they are missing some important nuances but then the same will likely apply to all anecdotal stories.