I asked a really similar question about my Roo a few months back. She just turned one in May. Last summer and winter she was very furry (could have been puppy fur?) and she gradually got very bald thighs, stomach and a less dense coat this spring/early summer. Prior to this, she was spayed in late March and about the same time I switched to Costco puppy chicken mixed with a little Costco/Kirkland adult chicken/rice/vegetable. Nothing else is different.
We brought her in for a check and booster shots and I asked our vet about it. She didn't think it was thyroid related because there are no other symptoms. We did try to take a blood sample just to check but Roo was a very unruly patient. So no blood sample.
I'm curious to see if Roo gets fuzzier when winter comes.
It did coincide with the food change, although she seems to be doing well on the food and her spaying/hormone level change.