Mysterious reduction in fur at the base of the tail

[exlibris]

Hi,
My work colleague is a bit concerned about her cat, it seems to be thinning near the base of the tail. He/she hasn't been over grooming or scratching, and the flea treatment is up to date. Based on this photo can anyone suggest a possible cause? She's booked into the vets, but if it ends up being something pretty obvious that she can treat herself she'd prefer to do that.
Any thoughts welcome.
I hope I've attached this correctly.
Thanks

[fjm]

Could be a flea allergy if the flea treatment she is using is not completely effective, could be hyperthyroidism, could be so many things. I would encourage her to see the vet and get a diagnosis, which may require blood tests.

[Mollycat]

Wow and I thought my cat's hair was thinning a bit (which it is but you can just feel it not see it, and I'm concerned!)

This cat should definitely see a vet, sooner rather than later.