I have had issues with both thumbs. I've had injections in the right thumb when it became a trigger thumb, after having an IV in my left hand -- hard to believe but it happened-left hand IV caused flare in upper body and within three days caused right thumb condition to worsen to trigger thumb. had to have 2 injections before it calmed down. I say it's from the scalenes. My cardiothoracic sugeon agrees.

splints usually helps but didn't for me.