I lost a lot of hair over a 6-7 year period in my 20s, too. Big bald patches above both my ears and the crown of my head was so thin my scalp regularly got sunburned.
Iron levels and hormones all normal.
Diet made no difference, Harley Street specialists tried to sell me all kinds of eye-wateringly expensive lotions and potions (@ilovecherries I feel you on that one) and I hit the point of looking at wigs and toppers etc. too.
It finally hit me when I started to consider using minoxidil and the woman at the clinic said I had to use it twice a day, every day and if I ever stopped the regrowth would all fall out again. Oh, and if I every fell pregnant I would have to stop, because it's strong enough to kill a cat.
To KILL a CAT. And she wanted me to put it on my head twice a day?
F&^K that.
I didn't think I was stressed either, until I realised that a couple of years before I noticed the hair loss I'd actually been through a very traumatic set of experiences. Maybe that's what triggered it, maybe not.
But my hair has grown back in over time. It's nowhere near the mass of curls it used to be, but it's much better. And not thinking about it helped a LOT.
I've heard many times that the body works in 7-year cycles. Maybe this is one of yours.
Things I found were helpful:
Dying your hair a lighter colour, and with highlights, masks the thin-ness, because your hair colour doesn't show up a big contrast from the colour of your scalp. Thinner patches are less noticeable with highlights because there are already different colours through the hair.
Talk to your hairdresser about cutting it in a style that has texture and movement - again, masking the flat stark hair/no-hair look.
If you want to go for a wig or a topper, go for it –there's absolutely nothing wrong with them. But make sure your relationship with yourself and your self-esteem is where it should be, so you're making that choice from an empowered place rather than a place of shame.
Hippy though it might sound, I really do think that not stressing about my hair loss and choosing to meet it where and how it was, made a huge difference to it growing back.