How can I treat de Quervain's tenosynovitis?

Depends. Those with mild symptoms can benefit from protective splinting, nsaids, icing and stretching. More severe cases, such as those seen after trauma or durning/after pregnancy usually don't respond to anything other than a corticosteroid injection. I treat many conditions with corticosteroid injections, but this is one of the few conditions that can actually be cured with one treatment.
Cortisone shot. Truly, a cortisone shot will be tremendously rewarding for anyone with dequervain's. I find that dequervain's is very painful for people. Get a cortisone shot!
Thumb Tendon. This is the tendon along the inside of your wrist that pulls the thumb up. Begin with heat, ice, wear a splint. We can also inject the tendon if this not respond to the simpler therapy. Use of the wrist and the thumb could cause this and can also make the pain worse. Begin with conservative approach and see how this does.