I have experienced the same problem for the last 15 years, on and off. I’m not aware of any difference in other external things that could be a cause. I have just automatically assumed it was the thyroid "weirdness" that I have experienced over the years. I was diagnosed with hypo at age 14, but developed hyper + thyroid cancer at age 43. I had Radiation treatment for the cancer and it was supposed to take care of the hyper, but now I fluctuate between hypo and hyper.