Please make sure your vet runs the whole thyroid panel, not just the T4. It's a better reading.
Symtoms? My Chloe is 3 and was diagnosed hypothyroid within 2 months of her racing career ending. All 4 races.
Prior to her meds, her T4 was .02 which is exceptionally low, even for a greyhound. She exhibted things like: being a spook to the point of never wanting to leave her crate except for eating and walks. She was frightened of everything.
She did have semi bald thighs. Other than that, not too many symtoms.
Now, she is a crazy maniac, fun loving party girl with most of the hair on her thighs grown back. It's hard to believe it's the same dog.
Hope your pup checks out fine.