This has also been a problem for me. I have always had thin, fine hair. After having 1/2 of my thyroid removed due a goiter growing on it, I was having all the symptoms one looks for for thyroid issues. My doc was testing my thyroid regularly, but because my levels were within the "accepted levels", she didn't think that was my problem....even tho' I only have 1/2 a thyroid now. When I asked to see my numbers from the previous years' testing, I was amazed...yes, I was still in the accepted range, but my numbers had jumped from extreme bottom end of the spectrum, to the upper extreme. Because I was still in that range, she thought I was ok. I asked if she would still think it was ok if my numbers had jumped that far from just inside the range to way outside. After checking with a thyroid specialist, she started me on thyroid meds, biotin, and women's Rogaine. My hair will never be thick flowing locks, but I am happy with how much I have now. I use Walmart generic rogaine, so reasonably priced...and I only use it 3 times a week now...just enough to keep the new growth from falling out.
So out of this long ramble, I guess what I am suggesting to anyone having their thyroid checked....ASK for your numbers!!!! And don't give up until you have answers and a solution that is right for YOU.
Patti