I used to have really thick and curly hair as a child.
Then over time, things changed. My hair became super dry and brittle, and started falling out a LOT. Interestingly, especially the lower layer at the back of my head was EXTREMELY dry and brittle.
Heavy metal toxicity does that, as the metals bind to your sulfur proteins. Hair is built from keratin which is a sulfur protein. Sulfur is extremely important in the body, not only for hair and nail health, but also for the joints and building enzymes.
Not only did I have a high load of heavy metals in my body, thanks to vaccines and amalgam fillings, but my diet was also lacking a lot of nutrients, and especially sulfur rich proteins. I became a vegetarian when I was 16 which in itself is not that bad if you know how to get your nutrients, even though animal proteins like eggs and fish are an easy way to get your sulfur rich proteins. But if you eat lots of legumes and greens, there should not be an issue. But I wasn't doing that.
My body got severely depleted in minerals and especially sulfur, as the heavy metals were binding to them.
The solution came with detox and taking sulfur. MSM (methylsulfonylmethane) is a great way to add some sulfur to your body. My hair had become already way shinier with taking Advanced TRS to detox heavy metals, but it got even healthier by adding in MSM, as well as silica and/or collagen.
If you start taking MSM, start slow. I used to get massive headaches from it but now can tolerate taking 5 g a day without problems. NAC (N-acetylcysteine) is also a good way to add in some sulfur but start very slow with this one as well, as it is a precursor for glutathione, the master detoxifier in your body, which can cause some detox reactions.
So if you are dealing with brittle and dry and thinning hair - look at toxicity levels in your body, change your diet to a more alkaline one and get in the sulfur.