This came up in my recommended books today. Haven't read it yet but can get it free on Kindle Unlimited. I always thought that (as well as lack of sunshine etc) low vitamin D was a symptom, not a cause of being hypothyroid. I do think it's extremely irresponsible to tout vit D as a way to lose weight - almost as bad as doing the same for T3.
However, there's a good illustration in the introduction that applies to plenty of bad thyroid research too. If you give a group of people a very small amount of water every day, they will eventually die, just as if they were given no water at all.
Conclusion, water is useless as a cure for dehydration!!!