My Boxer is suddenly aggressive to other dogs

[Arya]

My 17 month old spayed female Boxer (Echo) has started to be very aggressive to other dogs. All of a sudden, she is attacking other dogs at the dog park and starting fights. She has always been a 'reactive' dog compared to our other Boxer -- she guards the house, stares out the window and barks at people who walk by, and on a leash has never been able to ignore other dogs. She went to two rounds of puppy class and got her AKC Good Puppy certification, and has spent much of her life going to dog parks and never had a problem before.

I don't know what to do. Our male Boxer is super friendly (he's still 10 months old) and has no problem with other dogs, and I'm worried he's going to pick up on her reactive attitude. I'm no longer taking Echo to the dog park, but it completely ruins the fun that we used to have with the two. We would take them to the dog beach and the park to run all the time, and now I'm afraid to bring her out in public because she is so naughty.

She's also terrible on a leash. If we're walking and she sees another dog, she becomes uncontrollable. I used to use the Easy Walk Harness but I didn't like not being able to control her head while she was lunging at other dogs. I now use a Gentle Leader, and she completely ignores it and starts lunging and barking when she sees or hears another dog.

I'm at my wit's end. Aside from bringing in a professional trainer, what can I do???

[Sansal]

I think at this point you really do need to bring in a professional dog behaviorist to help you work through these issues. Another thing is to take her to the vet for a full check up. There are medical conditions that can lead to sudden behavior changes/ aggression. Low thyroid is one of them.