Cody is normally people-friendly when it comes to those that he knows, and lately he has been semi-friendly with strangers upon first meeting them. Two days ago, since we finally finished Cody-proofing the backyard by fixing all the areas that we had temporarily patched up, my mother let him and Moe outside to play and whatnot, and he was off-leash. When they were both ready to come inside, Moe walked in, but Cody was still standing in the doorway. She grabbed his harness to nudge him further inside so that she could shut the door (which she had seen me do occasionally) and he bit her arm a few times, drawing blood in 4 places. She was a little shaken and wasn't sure what to do, so she put him in his kennel until I got home from work, which was about 1 1/2 hours later.
He hadn't shown any signs of aggression before that, and hasn't since then. He seems to know that he was wrong; he walked around with his head down when I got home and let him out of his kennel, and he seems to show her more affection than before (well, his brand of affection with people besides me, which is mostly sitting at her feet, nosing/nuzzling her legs, and the like).
What I'm wondering is if this is an isolated incident or could it be a thyroid problem? From what I've read on here it seems like with thyroid there is change in attitude/behavior that goes on for a while, not just one episode, but I may be understanding that wrong. Do issues with thyroid start like that, gradually with isolated incidents here and there, or is the behavioral change more sudden?