Hair as become thick, dark, coarse and wiry over past years, with some bits left soft and smooth.

(I have labelled this as hair loss because it was the closest topic to my problem, but I haven't actually had any hair loss whatsoever).

Hello,
Over the past 3/4 years my hair (on my head) gradually become coarse, wiry, dry and very dark and very, very thick. I'm not just talking in the sense that a girl would say 'oh my hair is so dry', I'm saying my hair is so dry that it feels like a welcome mat by the front door and each strand of hair is so thick, they're probably about three times as thick as a normal hair that I've seen.  
I am 18 now, and this started when I was around 15.

My hair was always very soft, straight and of a light brown colour until about 3 years ago.

It actually looks very unhealthy, and one hairdresser even asked if I'd been treated for Alopecia because that's how it looked.
It looks extremely unusual, as each strand is so incredibly thick and has lots of 'kinks' (as in it curls and bends randomly and very sharply, not like normal curls) in it, it is also very dark, which looks strange because I am quite fair skinned and really do not suit dark hair, as I don't believe it's my natural colour.

In the past year it has actually got better, as in straighter and less frizzy, but it still looks very strange and it still has my confidence shot to pieces. People at school would call me 'carpet head' (they didn't realise it was really getting me upset, they were just joking around at the time).

I have no idea what the cause of this problem is, but I have just kept quiet about it until now because I thought I would just have to live with my new hair as it was just the way things were. However, now I am convinced it's not normal hair and that's there's something wrong with me.

One thing that's strange though is that there are small parts of my hair that are perfectly normal, notably the hair by my temples on either side as it grows from my parting towards my forehead.
In these parts the hair is soft, straight and feels very silky, as thought it was like my old hair. This hair has been there for as long as I can remember and hasn't changed in texture like the rest of my hair.

I used to straighten my hair, but haven't in about 2 years and I only use shampoo + conditioner and nothing else, no unusual products.

I have booked a thyroid test to see if that could be related but I am not sure because I haven't had any hair loss whatsoever, but I have had symptoms like depression and fatigue.

It may we worthy to note that about 3 years ago, before my hair became like this, I did dye my hair. However, if there was any damage from that I would expect it to be fixed by now?

Any help would be greatly appreciated, and I could get pictures if really needed. Thanks.

[Caryopteris]

Have you seen a dermatologist?  I'd talk to my primary care doctor certainly.  You may need hormones checked, a check for celiac disease, thyroid disease, not sure what else.  But it is true that our hair does signal other health problems a lot of the time.  If your diet needs improvement, you can immediately start eating more fruits, vegetables, organic meats/fish/poultry, and consider taking supplements.