Sarrasine had this (hence the less than flattering nickname of
"Barebutt") as a kitten and even as a young cat. She also had quite a
thin coat. Then about the time they discovered IAMS she went through a
moult and came out the other side with no bare bits and  a much thicker
coat. She was about 18 months old at the time. I don't know whether it
was the IAMS or not but she now has a good coat. (Redunzel has about
the silkiest coat I've ever felt on a cat- she's made for stroking and
luckily she likes being stroked)

Lesley

Slave of the Fabulous Furballs

have you been to a vet yet? the vet will do the looking and the
microbiology.
you can ask around. anybody else who has a cat with the same symptoms
in your area?

different parasites have different clinical signs. are the stools
normal or loose or even bloody or show white specs or what?

http://www.beaglesunlimited.com/beaglehealth_giardiasis.htm
this is for dogs but it's a start.

you can work with the vet and ask specifically for parasites if the vet
seems preoccupied. best if you and the vet work on this. both do
checking.

http://cats.about.com/od/parasiticdisease/

just something i found using "feline parasites" in google

some vets will dismiss parasites as impossible because:
1. that particular region does not have parasites in the soil.
2. the cat is kept indoors

these vets are being irresponsible and incompetent. an indoor cat in a
region not known for parasites can get parasites. less likely but not
impossible.

there are many different kinds of parasites. many, so it's difficult
for you to check up on all of them. you would have to read up on the
symptoms and then works out what lab tests are feasible and affordable.
some are cheap. i had a stool tested for $8. that was cheap. the vet's
office said not necessary. i said, do it and they did. i paid the money
and i wanted to make sure. sure it's unlikely but it does happen. and a
friend's cat had parasites, giardia, even though it's unlikely where i
live.

giardia can come through loud and clear via the tap water, even if the
tap water is purified and certified and chlorinated. a 1 micron water
filter, charcoal to add, takes out giardia. healthy people can take in
giardia. people with compromised immune systems cannot handle giardia
in the water supply.

this is just a suggestion. it's not easy. it's like a detective game.
you need to look at the symptoms. in the case of the friend's cat with
giardia, which, by the way, several vets had missed, the clues, very,
very vague, were the missing fur on the tummy region, the irritability
of the cat to the point where she had to be in the bedroom by herself
away from the other cats. the vet confirmed giardia. the cat was cured
and is furry again on her tummy and socializing with the other cats.
before the vet diagnosed giardia, i had guessed giardia or parasites. i
don't know why. maybe the missing fur triggered a previous case in my
memory.

you have to keep on trucking until you find the answer.

have you taken any stool samples to a vet? many vets will be bored and
tired and not want to exert their brain cells, so keep at it. just
changing the diet is an easy way to get rid of a patient or client but
that's not a real answer to a real problem. it's what vets say who do
not want to be bothered by a problem that is not immediate. some vets
are not good at difficult diagnoses which takes time and thought. this
is also true for all doctors, except maybe research ones, in fact,
almost all of the so-called helping professions.

http://www.marvistavet.com/html/eosinophilic_granuloma.html

My cat pulls the fur off of the back of her legs when she is overdue
for a steroid shot. This immune system disorder manifests as lesions
on the mouth sometimes, other times as "linear granuloma" or,
little itchy bumps in a straight line up the back of kitty's legs.
Easily controlled, by the way, and treatment is cheap.