Hair thinning. You suggest i should do?

Hello, sir! I'm 23 male. I've been experiencing hair thinning for the past 5 years. I've noticed that during that period of time, the first 2 years I experienced most of the thinning. But for the last 3 years, my hair has been pretty much the same i.e. no noticeable further thinning. I'm not bald at all but I have dry and thin hair on the top due to which, sometimes my scalp is visible. Plus I have a tiny but visible thin patch at the top back of my head. I want to add that I have had a medical condition in which I was diagnosed with deficiency of Vitamin B12 and D. Moreover I have and always had digestive problems throughout my life. Can these factors have an effect on my hair? How do I get the thickness of my hair back? What do you suggest I should do? This whole hair thinning problem has been very depressing lately. So please reply sir. Thank you!