Niacin likely won't do much. There is some evidence that fish oil might help - I'd suggest 1,000mg daily.
It would be worth it to have her vitamin D level checked, as well as her TSH (thyroid stimulating hormone), vitamin B12, folate, and CBC (complete blood count). That said, the lab most likely to evidence her depression is having her serum cortisol levels checked; cortisol is a stress
-related hormone that is elevated with depression. That said, cortisol levels are practically never checked because they are always high in depressed people - it's pointless for psychiatrists or other doctors to check to see if it's elevated when a patient is already saying they are depressed, because we already know it's going to be elevated; patients dislike giving blood for a lab result we already know is going to be abnormal. Checking her cortisol level might be worthwhile if your daughter were unable to communicate for whatever reason (autism
, etc.) and you were unsure if she was depressed, but you didn't report that as being the case.
It's also well worth your while to see if her depression is related to the timing of her menstrual cycle.
Studies show that psychotherapy or medication can help depression, and that both are more effective than either option alone. Exercise can also help.
One last thought: While it sounds as though your daughter can speak freely to you, it might be worth it to specifically ask her if anything has happened to her - such as a rape - about which she may be fearful of speaking. I have seen an awful lot of teenagers and young women who have told me that their depression started after a rape (or something else traumatic) that they felt they couldn't talk about.
I hope this helps, and that she gets some relief soon.