Hi Yasmin,
A good place to start is with a visit to your GP or, even better, your dermatologist. If you don't have a dermatologist, your GP will likely run a few basic blood tests while you wait for your referral, such as thyroid, ferritin, cholesterol, etc. Your GP may also suggest that you start using topical minoxidil (Rogaine/Regaine) in the interim. Your dermatologist will take a complete history, run additional blood tests, and will examine your scalp with a dermatoscope.
List of blood tests for women with hair loss:
https://donovanmedical.com/blood-tests-womenIn order to find an effective treatment, you need to know the underlying cause of your hair loss. There may be multiple factors that will all need to be addressed.
I suffer from female pattern hair loss triggered by hormonal imbalance. I've regained most of my density and my hair looks normal now. My current treatment is 5% minoxidil foam (Regaine/Rogaine) and Finasteride 5 mg (DHT blocker), as well as systemic MHT (oral estradiol/micronized progesterone).
Female pattern hair loss (FPHL) is very common. It causes miniaturization of the hair follicles, the hair becoming thinner and sparser. More info here:
https://static1.squarespace.com/static/5243dccde4b08fd9e4fc92ef/t/600e5c8ed9c8e22e3fd1e3c4/1611553934641/fphl+2021.pdfThere are so many potential causes of hair loss that you need an accurate diagnosis and you have to be patient with the process.
Some forms of hair loss are scarring, which means they result in permanent hair loss. That's why it's important to see a dermatologist.
You've been on and off hormone therapy. My hair follicles are very sensitive to hormonal changes and that would trigger telogen effluvium (temporary hair loss) in me. Every adjustment to my hormone therapy has resulted in temporary shedding. Anything that has the potential to alter the hair growth cycle can trigger shedding. So every hair loss treatment that I've trialed has caused temporary hair loss to some extent. That's why you have to stick with a treatment for many, many months before you can determine whether it's working.
There's a lot of snake oil and snake oil salesmen out there. For example, do not take mega doses of biotin. Thyroid dysfunction can cause hair loss. Large doses of biotin can interfere with blood testing.
Keep us posted and let us know what works for you.