I also have a 12 year old daughter who has just been diagnosed with depression.  We have just started treatment with a therapist as well as on Prozac.  I tried for several months to try and figure out if this was just "normal" pre-teen angst and hormones raging or if this was something more.  We went to the doctor and did full blood test analysis.  I would mention, if you do this, to have them look at the thyroid in the blood analysis.  A test that they do not standardly do is for Hashimoto's Disease which is another type of thyroid dysfunction.  You must request this be tested.  Hashimoto's can cause depression-like symptoms.  My daughter's health and blood tests turned out fine.

For us, the straw that broke the camel's back was the fact that my daughter couldn't get out of bed and started having stomach/nausea problems.  She was unable to get to school for 3 weeks.  The depression finally hit a physical manifestation.  She also started not being able to sleep and/or having long bouts of sleep and also isolated herself in her room and didn't want to go out and do anything.  She often talks about killing herself and thinks that life is meaningless.  In addition, she feels she is at fault, and apologizes often for what is happening.

I know that everyone is very different...and each has a road unto his own.  I hope, by sharing our story, that will help you gage your experience with your daughter.  Pre-teen is not a good time - we all know that.  I think perhaps it is the extremes we must look at and how the kid is dealing with them.