I have experience of it...look up telogen effluvium. I found the shampoos for thinning hair including Plantur not only didn't help, the ones intended to give more body actually made my hair rougher and harder to brush, seeming to pull more out. I have used special hair supplement vitamins to no avail. I did do loads of research and apart from Regaine, nothing seems to work. In many cases, the hair grows back of its own accord and I think this leads people to belive that whatever they were using at the time helped but the objective research says otherwise.
Now, some 9 years after mine started to thin, I just live with it. Growing it to waist length from a short crop hasn't made any difference to my bald spot and there are worse problems in the world. It does annoy me though that its "OK" for blokes to be bald but "not ok" for women unless you are doing it for fashion and look fabulous.
Sorry I haven't better news.