My experience is that people usually say teşekkürler/teşekkür ederim. When people ask you how you are it's sometimes just a way to open a conversation, it doesn't always mean that they necessarily want to know how you are.

Sometimes I just say "boş ver" or "sorma". Of course that's only when I know the person well, otherwise they might feel offended

When I call my family in law and I ask them how they are and they say thank you it annoys me greatly. Then I have to go "no, how are you really? I'm curious to know how things are going"

I pretty much hate "nasılsın?"

hi everyone,

here are some examples of "işte"

-neden yaptın? (in instance you ask to a child
-işte.. (the child doesn't want to answer, or he-she doen't have an answer

-partiye niye gitmedin?
-işte.. (he/she doesn't want to tell the reason or he/she doesn't know what to say)

-nasılsın?
- (eh) işte.. (he/she doesn't feel good, and doesn't want to talk about the reason, probably hopeless)

-kitap nerede?
-işte (there it is or here it is)(he/she points out the book and shows where it is, or maybe handle the book)

-anladım! sen bu elbiseyi almak istiyorsun
-işte ben de öyle diyorum zaten. (he/she is happy that one could understand what he/she tries to tell)

i hope i could explain it...

I also often hear the answer:

İşte, ne olsun (well nothing special, should there be?)
İşte, ne olsun, napıyım? napalım? (its not that good or special but what can we do about it)

Teşekkürler as an answer in my experience equals to being ok. I tried a few times to answer with Teşekkürler instead of saying that İ was ok.. and then it goes like this:

A - Nasılsın?
DK - Teşekkürler, sen nasılsın?
A - Ben DE iyiyim, sağol!

They often reply with 'I am good TOO' , though I didnt say that I was good. From this I got to the conclusion that thanking must be equal to being allright if not used with a negative response.

I usually just say that I am good, or like today when I am a bit ill, fena değilim.

Rahatsızmısın?

And then there's one of my favorites:

"İyi diyelim iyi olsun."
(Let's say "good" so that it may be good.)

i.e. it's not but if we say "good" maybe it will be...

Answering with "teşekkür ederim" is completely standard and acceptable; I hear it 100 times a day. I agree with the earlier poster that it may be interpreted as "I'm fine." I've never seen anyone offended at it or interpret it any way other than "[fine] thanks for asking." If they want to tell you they're not well, they'll find a way! (Even if they don't come out and say "kötü.")

"Rahat yaşayan rahatsız olur". ~atasözü