In reply to gingerkate:
Apologies if I'm repeating stuff other people have said; not read whole thread.
I have Raynaud's. It's distinguished from just getting cold fingers by the fingers going blue or white, as the circulation pretty much stops. Often only in the end section(s) of a digit.
It's a neurological problem that causes constriction of the arterioles (small arteries). It's exacerbated by stress, and isn't necessarily dependent on ambient temperature.
My mum suffered from it too, and was offered surgery to cut the sympathetic nerves. Unfortunately, this has frequent side-effects, such as the inability top regulate temperature of extremities, and sweaty palms. Not sure I'd recommend it. Hers stopped when she started taking blood pressure tablets.
I've found that keeping hydrated (to keep the blood thin), and maintaining food intake help to ease it. And keeping the wrists and forearms warm to help keep the blood vessels open; extermities will close earlier if cold.
I don't take any supplements, but things such as gingko biloba are sometimes recommended for assisting circulation, especially in smaller blood vessels (e.g. also suggested for tinnitus, on the basis that improving blood flow to the ears may help).
Oh, and a small amount of alcohol may help, being a vasodilator. (Incidentally, I found an abstract of some DoD research that suggested alcohol doesn't increase the risk of hypothermia.)
If your thumb is just numb, I'd suggest it isn't Raynaud's.
And yes, it does increase the risk of frostbite and chilblains.