Well, Systematic Estradiol definitely increases hair growth and density for some women, transwomen and men.
We just don't know all the factors in why Systematic Estradiol increase growth so we don't understand (yet) why it doesn't work for everyone (did they not get their blood levels high enough or were they perhaps too high). I suspect there is a sweet spot or goldilocks zone for most of us and it varies by individual. Generally, for women, a Blood Serum Level of 200-400 pg/ml will give more hair growth and density. I don't know what that number would be for men because of course there is no research, no clinical studies.
Both Phillip Kingsley and Belgravia seem to be proponents of topical Estrogens and Progesterone for hair growth.
I think the research in the form of clinical studies is just not there because everyone (Doctors, Pharmaceutical Companies, etc) are afraid of Estrogen ever since that incredibly flawed WHI Study in the early 2000's which suggested that Systematic Synthetic Estrogen and Synthetic Progestins were linked with increased Cancer and Heart Disease. It was an incredibly faulty Study and set the research in Estrogen & Progesterone Replacement research back by decades. Here's some data if you are interested in how that all came to be
https://www.womens-health-concern.org/help-and-advice/factsheets/hrt-the-history/
It's good to remember that Bio identical Estrogen (Estradiol and Estriol) are extremely safe in healthy amounts. Both women and men have endogenous Estrogen circulating in their bodies. Synthetic Estrogens are where the waters start to get murky. Estrogel, Divigel, Sandrena Gel are all Bioidentical Estradiol.
I really see no potential harm whatsover using an OTC Estriol, Estradiol or Biest cream on the scalp.
In fact, I'm wondering if OTC E creams might actually work better applied to the scalp (even though they are in much weaker strengths simply because they may hang around the follicle area for longer due to their poor systematic absorption). Pharmaceutical Estrogel is formulated to absorb through the skin and into the blood stream quickly so I'm wondering if even though it is much stronger it doesnt get to spend as much time saturating the follicles. It's just a hypothesis and probably somewhat flawed but maybe.
As a woman, I wish it were that easy to increase my Systematic E with OTC creams! I need the big guns (Estrogels etc) in order to raise my E level.