ginnyjm,
https://www.mskultrasoundinjections.co.uk/thumb-osteoarthritis-cortisone-injection#:~:text=Pain%20is%20very%20subjective%2C%20and,lasts%20for%20a%20few%20hours.
I'm having both thumbs and a few finger and wrists from bracing myself during a fall. The chiropractor can adjust the joints and I'm pain free for a few days. I'm considering cortisone injections, but they can cause degeneration of the cartilage.
An xray of the joint would show arthritis or gout, but won't image the tendon, but then you'd know.
Unless you recognize that you've been overusing the thumb with some task or damaged it like I did with a fall, you'd just as well not wait to see a doctor.
It's impossible to pick anything up without that opposing thumb.