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You might try taking this test-
Psych Central - Depression Screening Test

Depression for many of us doesn't need any external or situational stimuli. It can just happen out of nowhere. It is not as simple as a chemical imbalance but I think biology and bio chemistry play a big role. At least for me.

The symptoms you list are concerning. Especially thinking about suicide obviously.
Suicidal thinking
Lack of interest in things that normally interested you
Not wanting to be around people, isolation
Feeling down
Feeling numb
Low energy
Low motivation
All classic symptoms of depression.

It doesn't mean you have it for sure. Problems with thyroid, messed up hormones, vitamin D deficiency can cause it. I would never say someone having suicidal thought is just a moody teenager and chalk it up to that. I think you should take it seriously.

It would be best to get a thorough evaluation by professionals.

My daughter had it really bad when she was a teen and even had an attempt. At nineteen she started coming out of it and now at 21 is doing great. I don't think that means she is out of the woods for life. I still see her struggle with anxiety a lot and depression could hit her again. She has my genes.