I am midway through this situation.
In short I've been fatigued since I had lots of surgery in 2016/17/18. I also have an elevated heart rate for the last year
It was assumed to be peri but nobody tested Vit D, Folate, B12 etc. I was avoiding HRT (early breast cancer history) so it just seemed to be how it was and nothing to be done so I put up with feeling unwell constantly.
Started HRT last year in desperation as I felt so poorly. HRT helped with some symptoms (anxiety/ overwhelm/ joint pain) but the fatigue and elevated heart rate remain.
I've just had D, Folate, Iron, ferritin measured (amongst others) and my D (53.9) & Folate are both low, B12 is debatable. I've been taking a 1000iu D supplement since Feb anyway so who knows what it would have been without that.
I also have an odd thyroid issue which might be being made worse by the low vitamins or the thyroid might be effecting the vitamins 🤷♀️
Personally if it was starting this again, I would get bloods done for D, Folate, B12, iron, ferritin AND I'd get estradiol, progesterone, testerone and SHBG before I started HRT to understand the baseline. If you may have a thyroid issue I'd do TSH T4 and T3. This will give you an absolute baseline before you start adding anything. You will probably need to pay for these.
I didn't do this and I'm now trying to work out what is causing what. Taking HRT sends to have triggered some changes or is really coincidental. I really wish I had the pre HRT baseline to compare.
Sunday Girl know her stuff about this.
With a D level as low as yours, your GP should prescribe a loading dose.
It's been suggested to me that low D and Folate could be making me feel as shocking as I do so I'd get baseline bloods. Take the loading dose, also take a good B supplement if those are down and then see how you feel.
HRT definitely helps me but I'd have done it differently if I knew then what I know now.