Nina is a 85lb spayed, 4 year old boxer. The fur on her back is thinning in certain areas, not uniform in shape, now its spreading down her sides. It is very visible and now appears as though her skin is getting darker. She gets about 1.5 hours outside each day. She doesn't seem to be itching. I took her to the vet and he drew blood for a thyroid test, resulting in a 1 level, the lowest in the acceptable range. He also took a skin scraping which revealed no parasites. She seems to be normal, eating and drinking well. Any ideas as to what it may be or what I can do would be very much appreciated! Thank you.