Some years back, when I was depressed and tired all of the time, a pdoc tested my vitamin D level. It was normal.

There was a time when I was on a supplements kick. I took a lot of OTC fish oil, vitamin B complex, Biotin, Zinc, and about two or three other things, plus a small multi-vitamin. They did nothing for me at all. Some I took in hopes to reduce hair loss from Lithium or to reduce my triglycerides. The piles of pills were extreme. Not just the supplements, but my bipolar cocktail was huge, plus I took a couple other prescription meds. Taking so many pills felt psychologically distressing, so I cut out all of the supplements except the small multi-vitamin. Within weeks, I oddly started feeling a little better and my hair loss from Lithium eased. Go figure!

I have had to take a prescription iron supplement in the past because of iron deficiency anemia (from my IUD). It did seemingly help. Then the IUD was removed, so the prescription iron was stopped. I do still take OTC iron during my menstrual periods, when they are heavy. I think that's beneficial.