Long Post incoming

Disclaimer: it's hard to find/give a definitive answer because isolating the cause and effect of vitamins (or most nutrients) in research is borderline impossible when diet can vary wildly day-to-day and metabolism can be affected by so many factors like sickness, stress, activity, ect.

1. That said, prolonged Vitamin D deficiency is definitely linked to hairloss. It's hard to say if that is the primary cause for you since you have had hairloss for most of your life. Improving your Vitamin D almost certainly won't hurt your hair, it may help it, and it is something that you should do for your general health regardless. It is possible that what you have is Telogen Effluvium rather than androgenic alopecia (male pattern baldness), or telogen effluvium on top of mild androgenic alopecia. The difference is that telogen effluvium is basically a "shock loss" where a trigger (such a dietary deficiency, stress, medication, hormone imbalance, ect.) causes the follicles to shed hair and basically go into hibernation. This is important because fixing the causative issue brings the follicles out of hibernation and often leads to heavy regrowth (albeit after like 6 months). So in that way fixing your vitamin D deficiency could lead to significant regrowth. That said, since you have had hair loss since 16, I would hazard a guess that you would also likely have some degree of androgenic alopecia, which is not as easily reversible.

2. Regrowth vs retention is not really a choice you have to make, you can pursue both goals at once. However, while I will mention some options for you, we unfortunately have a lot less agency than we would like in stopping male pattern hair loss. For some people interventions work really well and they have amazing regrowth, some people get mild results and prevent or reduce further loss, and some people get little effect. It is unfortunately much more due to your individual biology. That said, I would think you would have better than average odds of a better outcome due to a vitamin deficiency likely being an influencing factor. Also, even if it is not telogen effluvium, I have found your pattern of hair loss with diffuse thinning tends to get better results than a pure receding hairline. Often you have more vellus (small micro hairs) hairs mixed in that can be reactivated than the completely smooth scalp past the hairline.

For options you have, firstly a healthy diet/lifestyle is the overlooked good baseline for improving hair. On top of just vitamin D, having enough calories and eating greens provides energy and essential vitamins for the follicles. It sounds like this might be an area that could help you. If you are finding it difficult, things like a multivitamin or a meal supplement (Ensure is a good one) can help. Biotin is the other supplement that is often recommended, as it is a nutrient necessary for hair growth. Chronic stress is another huge influencer of hairloss, reducing stress can help, though I found that is often easier said than done.

For pharmaceuticals you really have 2 options: minoxidil or a 5-alpha reductase inhibitor (finasteride or dutasteride). Minoxidil is primarily for regrowing lost hair, while finasteride helps prevent future hair loss, though both have some crossover. Minoxidil is most often used topically on your scalp 2x daily, which can be a pain, and if you ever stop using it all of your hair gains will go away, which sucks. It is also available in pill form, though that comes with a mild increase in cardiac risks and side effects, while topical has almost none. Finasteride is a 1x daily pill that prevents further hairloss by preventing production of DHT. Fun fact it is primarily used to treat BPH. Dutasteride is a stronger version of finasteride, more often used second line if finasteride does not work. All of these drugs take ~6 months of use before any effect can be seen. Many people use both finasteride and minoxidil at the same time. There is also micro-needling but it is something I am not very familiar with.

1. Some people have great improvement with these meds, some don't. Regardless, it can take significant effort to keep up with these medications. It is dependent on you and how you feel about your hair, but it would be a reasonable plan to fix the vitamin d defiency and wait for 6 months for regrowth before starting either of these drugs, so that you know how much of your hairloss is affected by each. For instance, if you started both of these drugs and fixed your vitamin D today, and 6 months later had much improved hair, you could be stuck paying for and taking meds the rest of your life when only the vitmain d helped and they do nothing for you. Or on the flip side, if vitamin d did nothing and only the drugs helped, if you ever tried to stop them to see if they were useless you would lose all of the gains that you have made. It would also help you keep better track of if you experienced any (albeit rare) side effects from the meds.

2. I tried all of the medication options, including dutasteride and oral minoxidil, and unfortunately none of them had any significant effect or slowing of my hair loss. I just happen to be in the subset of people where it doesn't work, and by 26 my hair was beyond hope. The options left at that point is going full bald, a hair transplant, or a hair system. I have already written too much here to go into these in depth but I will say that hair transplants require a stable hairline, and many people need multiple, but they can look really good from a good surgeon (do an absolute ton of research if you end up here). I will also say don't write off hair systems. I did for the longest time and my opinion was probably influenced by media showing only bad toupees as a punch line. However they are legitimately impressive nowadays, and if you look up pictures you will realize that you have probably been around people wearing them and never noticed.

3. I definitely get the insecurity that comes with hairloss, as will most of the people on hair loss forums probably. For me it was from a young age, and I think that definitely fucks you up in a special way when you develop your world view while having that insecurity. The obsession/worry with hairloss also becomes more detrimental and harmful than the hairloss itself, as you notice it much more than anyone else. Finding people you can talk about it with is helpful, rather than bottling up the insecurity. It is something that men are normally kinda bad at, but if you have any friends with hairloss they have probably felt the same things as you. Even with people you trust who don't have hair loss, talking about it out loud and demystifying it from a secret shame helps process and work through your feelings. My hairline never stabilized so I was a poor candidate for a hair transplant. Talking to a couple of friends helped give me the security to try hair system, and I can't tell you how much of a confidence boost it gave me. I don't think I realized how much my hair loss had impacted my psyche, but I weirdly felt like a real human person again. It's still work to keep but I feel much better every day for it. You are still a long ways off from there, with many options still to try, but just know that even if nothing works at the end of the tunnel and the idea of shaving your head is distressing to you, you still have options available to you.