Thin hair

Our recently adopted rat terrier, Millie, has an unusual problem with her coat. She's white with black markings (circles, patches) and anywhere that she's got black hair, she's nearly bald. The hair looks like its broken, not fallen out, though maybe some has fallen out too. Wherever she's white her coat is substantially thicker and healthier. Anyone have any ideas? Our vet doesn't know what could have caused this. He mentioned thyroid, but thought that would affect the entire coat. She was a rescue form a shelter so I have no history on her. She's negative for mange and otherwise seems very healthy, though she was a bit thin when we got her.
I'd appreciate any input! :D
Thanks!!
BTW, she's approx. 3-5 years old

A BIG thank you for rescuing! I am surely grateful for people like you! Has she been tested for thyroid? If she was seen by the vet & her tests came out negative then it probably has something to do with her previous home. Possibly mal-nurished or allergies. If she is having no other symptoms I would just put her on a great quality food & watch her closely. It sounds like it should clear up within a few months. The best of luck.

No, her thyroid hasn't been tested and it doesn't appear to be allergy related (our toy fox terrier has BAD allergies so we're familiar with that :rolleyes: ). We have her on a premium Venison and Rice diet (due to Chester's allergies) and she seems to be otherwise healthy and is HW neg and now on prevention. It just seems so unusual that its only the black hair. :confused:

Please look at this web site. www.sport-doberman.com/alopecia.htm This next phrase was pasted from it. " Black-hair follicular dysplasia is a rare hereditary
skin disease reported in mongrel and purebred dogs with black spots, and in the cow and horse. Only black areas of
the coat and underlying skin are affected." Good luck. Inform your vet. ;)

Thanks for the info!