Hmmm it’s hard to say. I recently lost 30 lbs on the Atkins diet - not particularly rapid loss - maybe 2 lbs or so a week. But my hair went shi**y too. Lots of fall-out and just generally dry-looking and frizzy. So I googled it and there are lots of threads about that. However, on a lot of those low carb forums they also linked to other diet forums (Weight Watchers, low fat etc) where people were complaining about the same thing. It seems that either rapid weight loss and / or a drastic change to the diet (whatever diet you follow - it’s the shock to the system that seems important) is just a bit of a shock to the system and can cause hair loss 3 - 4 months into a diet (as it takes that amount of time for a hair that’s stopped its growth phase to actually fall out). Generally, though it’s accepted that that hair will grow back within 6 + months or so (barring any underlying nutritional differences).
I suppose the question is was this hair loss all over or in the MPB areas (temples, top, crown)?
Did you have any hair loss prior to your diet?
The timing (4 months) suggests a diet-related loss, as does the suddenness and severity, which is good news as it should grow back. But if you had hair loss before, I dunno.
I’d ride it out for a few months and see what happens. Maybe supplement with a good multivitamin and mineral and a good B Complex vitamin (Maxi Hair, or the like) and see what happens. And using Nizoral shampoo 3 times a week couldn’t hurt, either.
However, whatever Hangin says, if it IS MPB then the vitamins won’t do a lot to address the issue - it’ll just make your remaining hair in the safe zone look healthy. If it’s MPB and not diet-related then you’ll need to consider actual meds - Minox, Fin etc.