Hyperthyroidism is pretty common on cats and could explain the weight loss. It could be her teeth too, our little kitty was very thin when we got her from Battersea and had about half of her teeth out while she was there. The nurse said that they tend to not groom themselves very well when their teeth are at them which could explain why her fur isn't in great condition.
Yes mine do too: she used to weigh 4.8 kilos, but last summer she had gone down to 4.3 (I think) and the vet was pleased. (we had been using a measuring scoop for her biscuits and not giving her any tinned meat as the vet suggested)
I can't take her to the vet tomorrow as I am at work all day, but was hoping to get a weight in the mean time.