I’m 53 and stopped dying last year — my last single process was March 2019 and I last had highlights (foils) in June 2019. I’ve always had fine hair, but a lot of it, and like you, I always felt like the dye helped make it thicker. I don’t have the thyroid issues you have, but it takes a while to really see what your natural hair is going to look like and do, At my last haircut appointment, the stylist said my new growth feels thicker to her than my highlighted and dyed ends. Her take is the chemicals can thin the hair shaft. I’m not objective enough to know yet if I see what she sees, but I will say that I have more regrowth around my hairline than I did when I was putting dye on my head every 5 weeks. I only started noticing the regrowth in the last couple of months, so again, I think it takes a lot of time to see what the result is going to be.
My only experience with thyroid issues is with pets, not people, but I would think that getting the right dose of thyroid meds would help with your hair. What does your doctor say about that?