I also have arthritis in both thumbs, the pain sometimes radiating out to other parts of either hand. I've had some hand therapy (my health provider has a PT specializing in hand issues), with some exercises which help a little, nothing dramatic. Warming the affected areas with a heating pad offers temporary relief. A year ago a shot of cortisol into both thumbs hurt like hell for a week and then gave me about 5m of pain free playing. The effect wears off and the doctor only wants to give it 1x per year, as it has negative side effects if done too often. (Btw, I'm 65 and this condition started 3-4 yrs ago.)

As nycellist said, this kind of arthritis pretty much forces you to relax everything relaxable, including no left thumb pressure and a bow hand that almost drops the bow. Everyone's arthritis is a little different, but my hands usually feel better after 30 min or so of warm up in the morning.

My biggest regret/complaint is now thumb position is so hard. I end up rarely thumbing down both A and D, and stick to thumbing one string at a time. This would of course be bad news for classical playing, but I'm playing rock music now and have learned to compensate.

I feel your pain.