One of the OFA approved laboratories for thyroid blood results is in Canada, I think. I believe the website is www.offa.org.
I'm not a fan of water spraying dogs. It is milder than a lot of corrections, but sometimes you need a dog to accept water spraying as a positive thing! Anyway, getting in trouble for growling, barking, etc. can make a dog think even more aggressively--after all, that person or other dog got them into trouble before!
Read the article "Attention, Please!" at the link with my signature. You'll find it in alphabetical order by title. Read the articles on Aggression there, too.
It is possible for a dog several years old to have thyroid problems connected with aggression. My dog was 7 when she was finally diagnosed with thyroid deficiency, and man, was she deficient by then. We were astonished by all the things it had caused, that corrected on the medication. But we did have to test her blood repeatedly to keep the dosage right. Thyroid deficiency is very common in dogs.
Avoid the situations where your dog could show aggression. Every time she does it, the habit gets stronger. Get a different behavior specialist--ask your veterinarian to recommend someone locally. And keep the dog's life low-stress and low stimulation for at least awhile.
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47