Mini Schnauzer behavior change

Question
I have a 3 1/2 year female mini schnauzer that I has recently begun showing some very aggressive behavior.  It seems to have started when we got a male resuce mini schnauzer.  She is very dominant with him but has tolerated him.  We got the male about 7 months ago.  Since then she has begun to bark, aggressively growl and even attempt to bite people other than our immediate family when they come over to our home.  We have tried putting her outside when we have visitors but she barks and throws herself at the glass door the entire time they are over unless someone is with her.  We have tried putting her upstairs in one of the bedrooms - same reaction.  We have also tried putting her in a kennel. I don't know what to do anymore.  She is a part of our family - we have had her since she was 6 weeks old but my children are afraid to have friends over and so am I.  Is there any kind of training that would help at this point?

Answer
The first thing you should do is to have her thyroid level checked as hypothyroidism can cause aggressive behavior in some dogs.

Bringing in that second dog may have made her think she needs to assert herself more to insure that she is the dominant figure in the house. Get her into obedience classes for several months and work with her every single day. Also start practicing "Nothing in Life is Free" with her; actually every member of the household should practice these things with her. http://k9deb.com/nilif.htm

If all else fails, get the name of a good animal behaviorist from your veterinarian so that you can get professional help with her.