What causes hand swelling?

Multiple causes. This could be low thyroid, a carpal tunnel, an allergy, problem with veins, arthritis, afrozen shoulder, early shoulder hand syndrome, tendonitis, kidney problem. It's also possible you just slept on it and the swelling will go away by itself.
Lots ?Carpal Tunnel? Lots of different things can cause hand swelling. Don't forget that carpal tunnel syndrome is one very common problem people can have and it can be a cause or an effect of hand swelling.