My left arm had been sore and easily got pins and needles. B12 is a big factor in circulation... That may have something to do with the left hand troubles. Im not too knowledgable about B12, but its definitely not something that should be taken lightly. I would suggest going out and getting yourself some Methyl-B12 (its better absorbed than the generic cyanocobalamin b12.) Long-term b12 deficiency can lead to irreversible nerve damage, memory loss, dementia etc.
Are you vegetarian by chance? (a lot of b12 absorbed through food comes from meat)
As far as vitamin D... there is a vast variety of other vitamins that should be taken along with it, especially Magnesium. If the hand thing is muscle related, it is VERY important to balance the amount of vitamin D and Magnesium you are taking.
I'm not an expert on any of this stuff but I would try checking out Vitamin D council (website) they know their stuff, and they can answer questions about b12 and your thyroid also. Hope this helps!
I'm just wildly speculating here but are you involved in software development? I did it for a long time myself on a great many UNIX systems using many editors, each used the ESC key as the master Meta key. I had to change editors myself since I had crippled my left hand (right was fine) by constantly extending my left hand during meta function. Amazing what a simple thing, done many times can do. Just a thought.