Its not necessarily only due to the HRT, have you had your thyroid checked, my Dr told me that that can also cause it, also myself and my daughter have had it terribly the past few weeks, we always get it change of season, Doc also said stress can be a factor, mine comes out when I wash it, and I have so little as it is, my daughter has lovely thick hair and she is also shedding loads, had her bloods tested and all was fine so could just be part of the season/part due to meno/daily stress etc. There are some shampoos I have seen that one can use as well as tablets if it gets really bad, all the best.