What are the best nutrients vitamins to thicken hair?

B Complex: You would try B Complex, Biotin. It would be nice to find out what the cause is. It might be stress related , then you need to correct that. It might be related to multiple conditions. I would suggest finding out why and correcting the underlying issue not just medicating yourself.

B Vits,A,C,D,E,Mg &:: Many vitamins are needed for healthy hair, including B3, B5, B6, B12, biotin & A, C, D, E & zinc. While deficiencies can cause hair loss & taking these may prevent hair loss, they are not proven to restore hair loss in most, but are important for healthy hair. The most effective are E, D & biotin. See http://bembu.com/hair-growth-vitamins