Vitamin D makes the blues go away

My therapist suggested that I start taking vitamin D for depression, especially in winter. I don't have seasonal affective disorder, I just get sad all the time anyway. But vitamin D does a lot for your system.

"Researchers are discovering that D also promotes normal cell growth and differentiation throughout the body, working as a key factor in maintaining hormonal balance and a healthy immune system....What’s more, evidence from studies tracking the prevalence of disease by geography and nationality shows clear links between vitamin D deficiency and obesity, insulin resistance, heart disease, certain cancers, and depression."

People all seem to have a vitamin D deficiency. Maybe it explains a lot of things like the diseases listed above. So should I start taking vitamin D? It is a hormone, not a vitamin, so will it interfere with the other drugs I'm taking? Will it do me any good? My therapist says you can tell a difference right away. I suppose I might as well give it a shot.