Ooooh Meg, so very sad to hear Avie has not been feeling well. I went through something similar in Jan. with Zander, vomiting then the d, some fever. I took him to the vet, he declared it a virus, gave him antibiotic, within a few days he was back to himself, phew!
Perhaps Avie had symptoms for a virus, however ... since they did the bloodwork it may have caught the thyroid problem early enough to where she may not be showing symptoms yet
Perhaps the vet can retest her thyroids within a few months?
I will join the team praying for her. Best to you & your baby.