Alopecia is the general medical term for hair loss. There are several websites about Alopecia, a simple google search will do the trick. For official websites in UK & US, visit Alopecia UK here, or the National Alopecia Areata Foundation here.
Some women lose hair because of conditions like hypothyroidism or hyperthyroidism. Visit the American Thyroid Association for more information. Read this article for some pointers on hairloss and hypothyroidism.