I was losing my hair a few years back the drs did blood tests and said I was fine. I went to see a tricologist (had to pay privately as they don't have them on the NHS) and when she looked at my blood results she said my iron was low.My hair just looks so stringy. Top all that off my hair is shedding and falling out as well, doctors have ruled out everything so god knows what's happening.
When we have blood tests the "normal" range for iron is between 15 and 300 which is a huge difference. My level was 30, which was a menstruating woman is low really. So she recommended I take 2 14mg iron tablets a day and it made such a huge difference, its taken 2 years but all my hair has grown back and is stronger, less breakage and looks so much healthier. Might be worth asking for a copy of your blood tests and doing some googling or seeing if you can see a tricologist. My appointment was £75 which is a lot of money but she used a camera on my scalp and did some other tests and now my hairs back its well worth it. Good luck!